Sahih Bukhari
Prayer at Night (Tahajjud)
Volume: 002 / Book Number: 021 / Hadith Number: 230
Narrated Al-Mughira:
The Prophet used to stand (in the prayer) or pray till both his feet or legs swelled. He was asked why (he offered such an unbearable prayer) and he said, "should I not be a thankful slave."