الْمُغِيرَةَ ـ رضى الله عنه ـ يَقُولُ إِنْ كَانَ النَّبِيُّ صلى الله عليه وسلم لَيَقُومُ لِيُصَلِّيَ حَتَّى تَرِمُ قَدَمَاهُ أَوْ سَاقَاهُ، فَيُقَالُ لَهُ فَيَقُولُ: أَفَلاَ أَكُونُ عَبْدًا شَكُورًا
It is narrated by Hazrat Mughirahra that the Prophetsa would stand [in supererogatory prayer] so long that his feet became swollen. Someone said to him, “Allah has forgiven you – past and future.” He responded, “Shall I not then be a grateful servant?”
(Sahih al-Bukhari, Kitab at-tafsir, Bab ‘li yaghfira laka Allahu ma taqaddama min dhanbika wa ma ta’akhkhara wa yutimma ni‘matahu ‘alayka wa yahdiyaka siratan mustaqiman’, Hadith 4836)
