Musannaf Ibn Abi Shayba 1:113

Abdullah ibn Numayr narrated to us, he said: Isra'il narrated to us, from 'Amir ibn Shaqiq, from Abu Wa'il, he said: I saw Uthman performing ablution (wudu), and he combed his beard with his fingers three times, and he said: I saw the Messenger of Allah, may Allah bless him and grant him peace, do it.