From Sahih Muslim (Hadith 4847):
It has been narrated on the authority of Jarir b. Abdullah who said:I saw that the Messenger of Allah (ﷺ) was twisting the forelock of a horse with his fingers and he was saying: (A great) benefit. i. e. reward (for rearing them for Jihad) and spoils of war, has been tied to the forelocks of horses until the Day of Judgment
Reference: Muslim 4847