Musnad Ahmad 482:137

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah narrated to us, from Abdullah ibn Hubayrah, from Abd al-Rahman ibn Jubayr, from Uqbah ibn Amir al-Juhani, that the Messenger of Allah (may Allah bless him and grant him peace) 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."