Musnad Ahmad 30:1490

Hassan and Yahya ibn Ishaq narrated to us, they said: Ibn Lahi'ah narrated to us, Abu Yunus narrated to us, from Abu Hurairah, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "If one of you applies kohl, let him apply it in an odd number. And if one of you cleans himself with stones (istijmar), let him clean himself with an odd number."