Musnad Ahmad 30:1556

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from Abu Yunus, from Abu Hurairah, he said: 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."