Sahih al-Bukhari 4837
صحيح البخاري#4837SahihNarrated Aisha:The Prophet (ﷺ) used to offer prayer at night (for such a long time) that his feet used to crack. I said, "O Allah's Messenger (ﷺ)! Why do you do it since Allah has forgiven you your faults of the past and those to follow?" He said, "Shouldn't I love to be a thankful slave (of Allah)?' When he became old, he prayed while sitting, but if he wanted to perform a bowing, he would get up, recite (some other verses) and then perform the bowing
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 65:358