Sahih Muslim Book 2, Hadith Number 460.

Chapter : While cleaning the nose and using of pebbles in the toilet, the odd number is preferable.

Abu Huraira reported: The Messenger of Allah (way peace he upon him) said: When anyone performs ablution he must clean his nose and when anyone wipes himself with pebbles (after answering the call of nature) he must do that odd number of times.