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)?”