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)? (Using translation from Muslim 2819b)
قَامَ النَّبِيُّ ﷺ حَتَّى تَوَرَّمَتْ قَدَمَاهُ فَقِيلَ لَهُ قَدْ غَفَرَ اللهُ لَكَ مَا تَقَدَّمَ مِنْ ذَنْبِكَ وَمَا تَأَخَّرَ قَالَ «أَفَلَا أَكُونُ عَبْدًا شَكُورًا؟»