Sunan Ibn Majah Book 1, Hadith 142
'Sniff up water into the nostrils thoroughly, two or three times.'"
"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.'"
"The Messenger of Allah said: 'Whoever performs ablution, let him clean his nose, and whoever uses pebbles to clean himself after defecating, let him use an odd number.'"