Musannaf Ibn Abi Shayba 1:279

Zayd ibn al-Hubab narrated to us, from Malik ibn Anas, from al-Zuhri, from Abu Idris al-Khawlani, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever performs ablution, let him clear his nose with water (istinthar). And whoever cleans himself with stones (istijmar), let him use an odd number."