Mishkat al-Masabih 4 Prayer كتاب الصلاة 636

وَعَنِ الْمُغِيرَةِ قَالَ: قَامَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ حَتَّى تَوَرَّمَتْ قَدَمَاهُ فَقِيلَ لَهُ: لِمَ تَصْنَعُ هَذَا وَقَدْ غُفِرَ لَكَ مَا تَقَدَّمَ مِنْ ذَنْبِكِ وَمَا تَأَخَّرَ؟ قَالَ: «أَفَلَا أَكُونُ عَبْدًا شَكُورًا»

Al-Mughira said that the Prophet got up at night to such an extent that his feet became swollen. On being asked why he did this when his former and latter sins had been forgiven him, he replied, “Should I not be a grateful servant?” (Bukhari and Muslim.)