The Prophet (ﷺ) used to stand (in the prayer) or pray till both his feet or legs swelled. He was asked why (he offered such an unbearable prayer) and he said, "should I not be a thankful slave."
Sahih al-Bukhari Book 19, Hadith 10
← Previous Hadith One night Allah's Messenger (ﷺ) offered the prayer in the Mosque and the people followed him. The next night he also offered the prayer and too many people gathered. On the third and the fourth nights more people gathered, but Allah's Messenger (ﷺ) did not come out to them. In the morning he said, "I saw what you were doing and nothing but the fear that it (i.e. the prayer) might be enjoined on you, stopped me from coming to you." And that happened in the month of Ramadan.

Sahih al-Bukhari Book 19, Hadith 9 Next Hadith → Allah's Messenger (ﷺ) told me, "The most beloved prayer to Allah is that of David and the most beloved fasts to Allah are those of David. He used to sleep for half of the night and then pray for one third of the night and again sleep for its sixth part and used to fast on alternate days."

Sahih al-Bukhari Book 19, Hadith 11