It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) had a horse, which he gave as a gift to a man from the Ansar. He used to hear its neighing; then one day he did not find it (did not hear its voice), so the Messenger of Allah (peace and blessings be upon him) said to him: "What has become of your horse?" He said: O Messenger of Allah, I have castrated it. He (peace and blessings be upon him) said: "Goodness is tied to the forelocks of horses, and spoils are bound to their forelocks until the Day of Resurrection; their forelocks are their warmth (strength), and their tails are their means of driving away flies."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7963