Maliks Muwatta Book 2, Hadith Number 2.

Yahya related to me from Malik from Abu’zZinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “When you do wudu, snuff water into your nose and blow it out, and if you use stones to clean your private parts use an odd number.”