Sahih Bukhari
Prayer at Night (Tahajjud)
Volume: 002 / Book Number: 021 / Hadith Number: 236
Narrated Abu-Wa il:
'Abdullah said, "One night I offered the Tahajjud prayer with the Prophet and he kept on standing till an ill-thought came to me." We said, "What was the ill-thought?" He said, "It was to sit down and leave the Prophet (standing)."