Musnad Ahmad 30:958

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Abu Idris al-Khawlani, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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."