The Messenger of Allah ﷺ used to pray until his feet became swollen. Someone said to him, "Do you do this when Allah has already forgiven you your past and future sins?"He replied, "Should I not be a grateful servant?"
كان رسول الله ﷺ يصلي حتى ترم قدماه. قال: فقيل له: أتفعل هذا وقد جاءك: إن الله قد غفر لك ما تقدم من ذنبك وما تأخر. قال: أفلا أكون عبدا شكورا.
Tirmidhi related it in Shamail (260) from Abu Ammar (Husayn bin Hurayth), who said, Fadl bin Musa told us, from Muhammad bin Amr, from Abu Salama, from Abu Hurairah, and he related it. Ibn Khuzayma (1184) graded it sahih and related it through the route of Fadl bin Musa in the same manner. Its chain is hasan because of Muhammad bin Amr, for he is truthful, but there are other routes for it.Tirmidhi also related it in Shamail (261), and Ibn Majah (1420), both through the route of Amash, from Abu Salih, from Abu Hurairah, who said, "The Messenger of Allah ﷺ would pray until his feet swelled up," as it appears in Tirmidhi's version, while in Ibn Majah's version it reads, "until his feet became swollen," with the rest of the hadith being similar. This chain is also hasan.Nasai (1645) related it through another route, from Sufyan, from Asim bin Kulayb, from his father, from Abu Hurairah, in a shortened form, "The Messenger of Allah ﷺ would pray until his feet cracked," meaning they split open. This chain is also sahih.These routes strengthen one another, so the hadith becomes sahih.