Musnad Abu Ya'la al-Mawsili 146:2

Abu Bakr ibn Abi Shaybah narrated to us, Ubayd ibn Sa'id narrated to us, from Shu'bah, from Abu al-Tayyah, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Blessing is in the forelocks of horses."