THE EARLIEST HISTORICAL SOURCES OF THE INCIDENT OF KARBALA
The Battle of Karbala (680 CE) has a central position in Shia history and the event is annually commemorated every year in the Islamic month of 'Muharram' (the first Islamic calendar month) by centrally the Shia and also by many Sunni Muslims.
However, as with many Islamic sources, the nature of this narrative as found in the historical record is not as well known by many Muslims who simply rely on the traditions as inherited and taught by their forefathers.
Despite being over approximately 130 years removed from the death of the Prophet at the time of his work, Ibn Ishaq (d.767 CE) remains the earliest source of the Prophet's biography that is transmitted today. However, much of the events that occur after the Prophet's demise such as the Battle of Karbala are learnt through the sources of later historians such as Al-Tabari (d. 923 CE) and not Ibn Ishaq.
Al-Tabari however, who at the time of his death was nearly 300 years removed from the Prophet's death and nearly 250 years removed from the event of Karbala makes use of traditions that have reached him from other sources.
Al-Tabari's collossal work entitled 'Tarikh al-Rusul wa al-Muluk' (History of the Prophets and Kings) remains a source of history covering the period up till the early 10th century. The English translation of this work spans 40 volumes including the index and is a major source of study for historians and academics.
However Al-Tabari's excerpt from the introduction of his work is worthy of note (bold emphasis mine):
"Let him who examines this book of mine know that I have relied, as regards everything I mention therein which I stipulate to be described by me, solely upon what has been transmitted to me by way of reports which I cite therein and traditions which I ascribe to their narrators, to the exclusion of what may be apprehended by rational argument or deduced by the human mind, except in very few cases. This is because knowledge of the reports of men of the past and of contemporaneous views of men of the present do not reach the one who has not witnessed them nor lived in their times except through the accounts of reporters and the transmission of transmitters, to the exclusion of rational deduction and mental inference. Hence, if I mention in this book a report about some men of the past, which the reader of listener finds objectionable or worthy of censure because he can see no aspect of truth nor any factual substance therein, let him know that this is not to be attributed to us but to those who transmitted it to us and we have merely passed this on as it has been passed on to us"
However Al-Tabari is not the source of the incident of Karbala. The earliest source is an Arab traditionalist by the name of Abu Mikhnaf who died in 774 CE who was writing at approximately the same time as Ibn Ishaq. Al-Tabari however is indeed the earliest source which makes use of Abu Mikhnaf's source comprehensively.
Though Abu Mikhnaf allegedly wrote many works (as noted by Shia historian Ibn al-Nadim d.995CE in his Al-Fihrist), Abu Mikhnaf's 'Kitab Maqtal Al-Husayn' remains the work through which we learn much details of the event in question. Abu Mikhnaf clearly had Shia interests and his grandfather was believed to have given his life at the Battle of Siffin which was allegedly fought between Ali ibn Abi Talib and Muawiyah in the year 657 CE.
No original work of Abu Mikhnaf survives and his work is only transmitted through his students and later historiographers such as Al-Tabari (d.923 CE) who have mainly derived their source of the information of the event from his work. Al-Tabari often quotes directly from Abu Mikhnaf but also quotes traditions through Hisham b. Muhammad al-Kalbi which source back to Abu Mikhnaf. It is quite possible that Al-Tabari also made use of a rescension of Muhammad al-Kalbi to source Abu Mikhnaf's work.
Some critical points about Abu Mikhnaf's work are keen to note:
"Abu Mikhnaf is not very particular about and scrupulous in authority chains. He has abundantly incorporated in his narratives, especially in the narration of Siffin episode, the tribal stories and the local gossips. For these reasons the muhaddith scholars regard his as a weak source. He has heavily relied on the tales of his own tribe. Some other Kufi traditions have also served as the source of his information" 
"Sometimes, he quotes lengthy dialogues in full, and cites complete texts of lectures or discourses. He does also use poetical compositions for the embellishment of a report and in this regards his narratives bear very close resemblance with the ayyam tales or the Qisas (stories)" 
With regards his sources, it is worthwhile to note that:
"Sometimes, he does not mention the name of the premier authority and quotes simply 'from someone who was present there', or 'from a member of the tribe of Banu Fazara', etc. He has also obtained his reports from women who had either participated in the happenings or were knowledgeable in the matters concerning the episode" 
However, well reputed Shia scholar S.H.M Jafri who remains in relative favour of the general authenticity of Abu Mikhnaf's report portrays his argument as follows:
"Gibb suggests that Abu Mikhnaf presents an Iraqi or Kufan, rather than purely Shi'i, point of view in his narratives.91 In this his sympathies are no doubt on the side of Iraq against Syria; for Ali, against the Umayyads. Yet in the opinion of Wellhausen there is not much of a bias noticeable, at least not so much as to positively falsify fact.92" 
Abu Mikhnaf's credibility as a source is further supported by his relative closeness to the event:
For this reason, in the Maqtal, Abu Mikhnaf cites his authority with the clear observation wa kana qad shahida qatl al- Husayn (and he witnessed the murder of Husayn). Without exception, throughout his narrative he uses the verb haddathani (he told me); and if his report is not directly from an eyewitness, he cites only one or two intermediaries who had received the account from the eyewitness himself. Thus in our quotations above concerning the statements of loyalty, pledges, and rajaz, the isnad runs:
1: Abu Mikhnaf—Muhammad b. Qays (eyewitness).
2: Abu Mikhnaf-Harith b. Abd Allah b. Sharik al-Amiri (eyewitnesses).
3: Abu Mikhnaf- Abd Allah b. Asim and Dahhak b. Abd Allah (eyewitnesses).
4: Abu Mikhnaf-Abu Janab al-Kalbi and Adi b. Hurmula (eyewitnesses).
5: Abu Mikhnaf-Muhammad b. Qays (eyewitness).96 
However, whilst discussing the source of the manuscripts, an appreciation of criticism is acknowledged by the Shia scholar and an attempt is made to contextualise the nature of the criticism against Abu Miknaf's work.
"Mention must finally be made of the four manuscripts of the Maqtal, located at Gotha (No. 1836), Berlin (Sprenger, Nos. 159-160), Leiden (No. 792), and St. Petersburg (Am No. 78). It was from the first two that Ferdinand Wüstenfeld made a German translation of the work entitled Der Tod des Husein Ben Ali und die Rache (Göttingen, 1883). Wüstenfeld, while convinced of the early origin of these manuscripts, doubts that the author was Abu Mikhnaf.100 The foremost argument he puts forward is that it contains some miraculous and supernatural types of stories, such as terrible manifestations of grief in natural phenomena: reddening skies, bleeding sands, and so forth. Ursula Sezgin questions Wüstenfeld's criticism at several points and suggests that while the existing manuscripts may be the recensions or rewritings made by some later unknown writers, the fact remains that Tabari's main source of Abu Mikhnaf was Ibn al-Kalbi.101
However, some of these miraculous stories or fantasies have found a place even in Tabari, which suggests that these might have been originally written by Abu Mikhnaf himself or may have been incorporated by Ibn al-Kalbi when he rewrote his master's work. But to cast doubts on Abu Mikhnaf's authorship of the Maqtal only on the grounds that some supernatural and miraculous events are recorded, as Wüstenfeld is inclined to suggest, would mean to ignore certain tendencies of the age. It would perhaps be a grave error to expect that a book written in the early eighth century about a great religious personality would not accept supernatural occurrences as a matter of course, especially when the main event itself is so charged with emotion and suffering" 
Abu Mikhnaf (d.774 CE) remains the earliest source of the Karbala incident captured primarily through the works of much later historians often centuries removed from the event. No original work of Abu Mikhnaf survives. Regardless of whether the source is authentic or contains stories that have been embellished over time to serve a particular sectarian bias arising from a powerful political milieu in the 1st century AH is a judgment for the individual to make.
What remains indisputable is the central position the narrative of Karbala occupies particularly in Shia thought and belief. Shia mourners in many places around the world express their grief to the tragedy of the event by weeping or tapping and in some cases beating their chests in an action that is known as 'Ma'attam'. In some cases, mourners even flog themselves with chains and whips often causing injury and bleeding to themselves.
 SINGH. N.K, Encyclopaedia Historiography of the Muslim World, Global Vision Publishing House, First Edition 2003, Page 130
 Ibid., Page 134
 JAFRI. S.H.M, The Origins and Early Development of Shi'a Islam, American University of Beirut 1976, Chapter 7, The Martyrdom of Husayn, Notes cited: Note (91) EI2 article "Abu Mikhnaf" and Note (92) Wellhausen, loc. cit.
 Ibid., Note cited: Note (96) See Tabari, index
 Ibid., Notes cited: Note (100) See Der Tod des Husein , Wüstenfeld's preface and Note (101) Sezgin, Abu Mikhnaf , pp. 190 ff