Musnad Ahmad 30:3587

Uthman narrated to us, Yunus informed us, from al-Zuhri, from Abu Idris, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever performs ablution, let him clear his nose with water (istinthar). And whoever cleans himself after relieving himself (istinja), let him use an odd number."