عن عائشة قالت كان رسول الله صلى الله عليه وسلم إذا صلى قام حتى تفطر رجلاه قالت عائشة يا رسول الله أتصنع هذا وقد غفر لك ما تقدم من ذنبك وما تأخر فقال يا عائشة أفلا أكون عبدا شكورا (صحيح مسلم #2820)
Hazrat ‘Aa’ishah (radhiyallahu ‘anha) mentions that when Rasulullah (sallallahu ‘alaihi wasallam) would perform salaah, he would stand for such a lengthy period that his mubaarak feet would (swell and) become cracked. Hazrat ‘Aa’ishah (radhiyallahu ‘anha) asked Rasulullah (sallallahu ‘alaihi wasallam), “O Rasul of Allah (sallallahu ‘alaihi wasallam)! Why do you exert yourself in this manner, whereas your past and future inexactitudes have all been forgiven?” Rasulullah (sallallahu ‘alaihi wasallam) replied, “O ‘Aa’ishah (radhiyallahu ‘anha)! Should I not be a grateful servant (by expressing gratitude for His countless favors through exerting myself in His remembrance and worship)?”
Note:
Rasulullah (sallallahu ‘alaihi wasallam) explained to Hazrat ‘Aa’ishah (radhiyallahu ‘anha) that it is only right for a servant to express appreciation and gratitude to Allah Ta’ala for whatever He has granted one. Through one exerting himself in salaah, which is the greatest of ‘ibaadaat and most beloved to Allah Ta‘ala, one will be able to earn His divine pleasure and express the loyalty of servitude. Hence, one can never ever fulfill the Haq of Allah Ta‘ala’s gratitude, but one can try to show Allah Ta‘ala that he is trying to please Him by standing in the dead of night when all are asleep.