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.
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 361
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 361