Musnad Ahmad 482:136

Hassan narrated to us, Ibn Lahi'ah narrated to us, from Abdullah ibn Hubayrah, who said: Abd al-Rahman ibn Jubayr informed me that he heard Uqbah ibn Amir saying: The Messenger of Allah, peace and blessings of Allah be upon him, said: "If one of you cleans himself with stones (istijmar), let him clean himself with an odd number. And if one applies kohl, let him apply kohl with an odd number."