Mr. Subhi Mansoor of Ahl AlQur'an argues strongly for Qur'an being sufficient, gives the following explanation for why prayer method, units. etc are missing in the Qur'an (He takes Salat as prayer only). An interesting point of view:
Thus, the one who believes in the Qur’an, believes God’s assertion that He did not leave anything out of the Quran and that it came down clarifying and detailing everything.
And just so that no one manipulates the Qur’an’s intended meaning with their evil desires or bad intentions so that they say ‘where is such and such a thing mentioned in the Qur’an?’ – they must grasp the concept of the Qur’an’s logic before they are quick to accusations.
The Almighty says “…We did not leave anything out of this book… (6:38),” tafreet means to omit or, leave out a crucial and important thing. For example, we don’t find a problem regarding the method for prayer or the number of its units. If God (The Knower of the past, future, and present!) knew that we would face a problem with regard to prayer, then He would have made clear to us its method, the number of prayers, and its times with detail. However, He only brought down the Qur’an clarifying what we actually need in the present and the future, He brought down the Qur’an with truth and with the law “God is the One who sent down the scripture, to deliver the truth and the law… (42:17).” There is no scope for addition for which we have no need. Had the Qur’an been revealed detailing for us the prayers while we already know what it is and have been practicing it since our childhood, then that would have been somewhat flippant; and there is no scope for flippancy in the Book of God “By the sky that returns (the water). By the earth that cracks (to grow plants). This is a serious narration. Not to be taken lightly. (86:11-14)”.
Thus, the Qur’an has not left out anything for which we have a need.
The Almighty says “…We have revealed to you this book to provide explanations for everything… (16:89),” Tibyaan means to clarify something which requires clarification and explanation. Yet, something which is inherently clear doesn’t require elucidation, lest any excessive explanations amount to curiosity and mere prattle for which we have no need.
God, Most Glorious, revealed His Book with straightforward verses in which is no scope for nonsense or addition. For this reason, the Qur’an clears up what it has to clear up, and everything that requires clarification and elucidation has been clarified and elucidated in the Qur’an. And something which doesn’t require elucidation is in no need to be elucidated, especially in a Book whose verses have been detailed from a Most Wise, Most Cognizant.
For this reason, elucidation in the Qur’an is connected to guidance, mercy, and good news for the Muslims, “…We have revealed to you this book to provide explanations for everything, and guidance, and mercy, and good news for the submitters. (16:89)”. Thus, the explanations of the Qur’an are a “guidance” for the one seeking guidance amongst a mass of ambiguity and uncertainty. The explanations found in the Qur’an are also a “mercy” for the one seeking guidance because it makes known to him/her what was unknown, and it guarantees him/her security and divine mercy as well as good news.
The details of the Qur’an are also connected to guidance and mercy, The Almighty says “We have given them a scripture that is fully detailed, with knowledge, guidance, and mercy for the people who believe. (7:52)”. Therefore, the Qur’anic details which include everything are a guidance and a mercy for those who need these details. And if it so happened that such things were clear and didn’t require detailing or clarification, then it would be futile to clarify what is already clear.
Exalted is God above futility!