Sahih Muslim
The Book of Purification (Kitab Al-Taharah)
Book Number: 002 / Hadith Number: 458
Abu Huraira reported Allah's Apostle (may peace be upon him) as saying: When anyone wipes himself with pebbles (after answering the call of nature) he must make use of an odd number and when any one of you performs ablution he must snuff in his nose water and then clean it.