Musannaf Ibn Abi Shayba 1:114

Waki’ narrated to us, from al-Haytham, Hammad narrated to us, from Yazid ibn Aban, from Anas, that the Prophet (may Allah bless him and grant him peace) said: "Gabriel came to me and said: 'When you perform ablution (wudu), comb through your beard.'"