Sunan Ibn Majah — Establishing the Prayer and the Sunnah Regarding Them — Hadith No: 1419
Translation
It was narrated from Ziyad bin ‘Ilaqah that he heard Mughirah say:“The Messenger of Allah (ﷺ) stood (in prayer) until his feet became swollen. It was said: ‘O Messenger of Allah, Allah has forgiven you your past and future sins.’ He said: ‘Should I not be a thankful slave?’”