A grateful slave
Aishah (radi Allahu anha) said: The Prophet (sal Allahu alaihi wa sallam) would stand (in prayer) so long that the skin of his feet would crack. I asked him, “Why do you do this while your past and future sins have been forgiven?” He said, “Should I not be a grateful slave of Allah?”
[Sahih al-Bukhari and Muslim]
1. All the Prophets were free from major sins. However, some religious scholars justify some of their minor sins with plausible reasons but the majority of them maintain that they are free from all sins because of their innocence. In this situation, it does not make any sense to speak of their sins. But the fact of the matter is quite different. Anything short of excellent done by them is counted as a sin.
2. The more one is rewarded by Allah, the greater the proportion of one’s gratitude of Allah for His Benevolence should be. The best form of doing so is that one should not only be highly obedient and dutiful in the performance of the obligations but also add maximum voluntary prayers to them.
And Allah Knows Best!