Sahih Muslim Book 39, Hadith Number 6773.
Chapter : The doing of good deeds very frequently and putting one’s best efforts in devotion to Allah.
This hadith has been transmitted on the authority of Mughira b. Shu’ba and the words are: Allah’s Apostle (may peace be upon him) kept standing in prayer (for such long hours) that his feet were swollen. They (his Companions) said: Verily, Allah has pardoned for thee the earlier and the later of thine sins. Thereupon he said: Should I not prove myself to be a grateful servant (of Allah)?