Sahih Muslim
The Book Giving Description of the Day of Judgement, Paradise and Hell (Kitab Sifat Al-Qiyamah wal Janna wan-Nar)
Book Number: 039 / Hadith Number: 6774
A'isha reported that when Allah's Messenger (may peace be upon him) occupied himself in prayer, he observed such a (long) qiylm (posture of standing in prayer) that his feet were swollen. A'isha said: Allah's Messenger you do this (in spite of the fact) that your earlier and later sins have been pardoned for you? Thereupon, he said. A'isha should I not prove myself to be a thanksgiving servant (of Allah)?