It was narrated that Abu Hurairah said:
"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.'" (DE)
English reference (DE): Vol. 1, Book 1, Hadith 409 (DE)
Arabic reference (DE): Book 1, Hadith 443 (DE)