used to offer prayer at night

Narrated Aisha (Radi-Allahu 'anha):

The Prophet (Sallallahu 'Alaihi Wa Sallam) 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.

Bukhari Vol. 6 : No. 361