From Sahih Muslim (Hadith 7125):
This hadith has been transmitted on the authority of Mughira b. Shu'ba and the words are:Allah's Apostle (ﷺ) 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)?
Reference: Muslim 7125