What is the source of Saitama's enormous power? I have seen only the anime and not the manga. I am utterly curious about the ability and power of Saitama.

In the starting episodes, he mentions that he trained very hard and also details his training regime. 

However, I am still curious to know: is there any mention of the source of his enormous power somewhere in the Manga?
The anime version of One Punch Man is really accurate on regard to the manga so nothing has been revealed in the manga that hasn't in the anime.

As you mentionned, in the manga, Saitama followed what he would call a hard training every day :


100 push-ups
100 sit-ups
100 squats
10 kilometers every day
no air-conditioner




As a result, Saitama loses his hair, and becomes incredibly powerful.



This training is the source of his power.
Keep in mind that One Punch Man is intended as a shonen parody as it is making a parody of its codes. 

In a lot of shonens, training is a main part of the manga. 

As an example, in Dragon Ball, the sayans spend years in the Hyperbolic Time Chamber in order to become stronger. 

By making the training of Saitama so simple (compared to the heroes capacities), One Punch Man is making fun of this Shonen code. Therefore we may not expect any more explanation on what Saitama did in order to become stronger.