Sahih Muslim Book 39, Hadith Number 6774.

Chapter : The doing of good deeds very frequently and putting one’s best efforts in devotion to Allah.

‘Aisha 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. ‘Aisha 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. ‘Aisha should I not prove myself to be a thanksgiving servant (of Allah)?