Musnad Ahmad 30:103

'Abd al-Rahman narrated to us, Malik narrated to us, from al-Zuhri, from Abu Idris, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "Whoever performs ablution, let him rinse his nose (with water and expel it). And whoever cleans himself with stones (istijmar), let him use an odd number."