Musnad Abu Ya'la al-Mawsili 146:6

Abu Khaythama narrated to us, Wahb ibn Jarir narrated to us, Shu'bah narrated to us, from Abu al-Tayyah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Blessing is in the forelocks of horses."