Sahih Muslim Book 3, Hadith Number 731.

Chapter : Ablution does not break by dozing in a sitting posture.

Anas reported: (The people) stood up for prayer and the Messenger of Allah (may peace be upon him) was whispering to a man, and in the narration of ‘Abd al-Warith (the words are): The Apostle of Allah (may peace be upon him) was having a private conversation with a man, and did not start the prayer till the people dozed off.