Sayyiduna Utbah bin Abd al-Sulami (may Allah be pleased with him) heard the Messenger of Allah (peace and blessings be upon him) say: "Do not cut the forelocks of horses, nor their manes, nor their tails, for their tails are their fly-whisks, their manes are their warmth, and goodness has been tied in their forelocks."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12903