Audhu Billahi minash shaitanir rajeem.
Bismillah ir Rahman ir Rahim.
39:18 Those who listen to the Word, and follow the best (meaning) in it: those are the ones whom Allah has guided, and those are the ones endued with understanding.
As for As Saum look up the follower of previous scripture since Quran is confirmation of previous scripture.
For christian it's Lent. 40 days of fasting. https://en.wikipedia.org/wiki/Lent
For Jews it's Ta'anit https://en.wikipedia.org/wiki/Ta%27anit