Sahih Bukhari | Volume 6 | Hadith Number 361
Narrated by 'Aisha
The Prophet used to offer prayer at night (for such a long time) that his feet used to crack. I said, "O Allah's Apostle! Why do you do it since Allah has forgiven you your faults of the past and those to follow?" He said, "Shouldn't I love to be a thankful slave (of Allah)?' When he became old, he prayed while sitting, but if he wanted to perform a bowing, he wound get up, recite (some other verses) and then perform the bowing.