Sahih Muslim Book 2, Hadith Number 459.

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

Hammam b. Munabbih reported: This is what Abu Huraira transmitted to us from Muhammad, the Messenger of Allah (may peace be upon him), and he mentioned a number of a Hadith, of which this is one: that the Messenger of Allah (may peace be upon him) said: When anyone amongst you (performs ablution) he must snuff his nostrils with water and then clean them.