Musnad Ahmad 723:2

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Mansur, from Hilal ibn Yasaf, from Salamah ibn Qays, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you perform ablution (wudu), then perform istinthar (expelling water from the nose). And when you perform istijmar (cleaning oneself with stones), then use an odd number."