Musnad Ahmad 30:1908

'Affan narrated to us, Abu 'Awanah narrated to us, 'Umar ibn Abi Salamah narrated to us, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you cleans himself with stones (istijmar), let him use an odd number."