Sahih Muslim Book 3, Hadith Number 733.

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

Qatida reported: I heard Anas as saying that the Companion of the Messenger of Allah (may peace be upon him) dozed off and then offered prayer and did not perform ablution. He (the narrator) said: I asked him if he had actually heard it from Anas. He said: By Allah. yes.