Sahih Bukhari Volume 006, Book 060, Hadith Number 360.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 60. Prophetic Commentary on the Qur'an (Tafseer of The Prophet (PBUH))
Narrated By Al-Mughira : The Prophet used to offer night prayers till his feet became swollen. Somebody said, to him," "Allah has forgiven you, your faults of the past and those to follow." On that, he said, "Shouldn't I be a thankful slave of Allah)?"