Sahih Muslim 2819 b
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)?
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَابْنُ، نُمَيْرٍ قَالاَ حَدَّثَنَا سُفْيَانُ، عَنْ زِيَادِ بْنِ عِلاَقَةَ،
 سَمِعَ الْمُغِيرَةَ بْنَ شُعْبَةَ، يَقُولُ قَامَ النَّبِيُّ صلى الله عليه وسلم حَتَّى وَرِمَتْ قَدَمَاهُ قَالُوا
 قَدْ غَفَرَ اللَّهُ لَكَ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ ‏.‏ قَالَ ‏
"‏ أَفَلاَ أَكُونُ عَبْدًا شَكُورًا ‏"‏ ‏.‏