Musnad Ahmad 30:612

'Abd al-Razzaq narrated to us, Malik narrated to us, from Ibn Shihab, from Abu Idris al-Khawlani, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you performs ablution (wudu), let him clear his nose with water (istinthar). And when one of you cleans himself with stones (istijmar), let him use an odd number."