Sunan Ibn Majah Book 1, Hadith 140
The Messenger of Allah said to me: 'When you perform ablution, clean your nose, and when you use pebbles to clean yourself after defecating, use an odd number.'"
"The Messenger of Allah came to us and asked us for water for ablution. I brought water to him and he rinsed his mouth and sniffed water up into his nostrils from one handful."
"I said: 'O Messenger of Allah! Tell me about ablution.' He said: 'Perform ablution properly and sniff water up into your nostrils, thoroughly, unless you are fasting.'"