The Messenger of Allah ﷺ said, "Do not cut the forelocks of horses, for blessing lies in them. Do not trim their manes, for they keep them warm. And do not cut their tails, for with them they swat away flies."
قال رسول الله ﷺ: لا تقصوا نواصي الخيل، فإن فيها البركة، ولا تجزوا أعرافها؛ فإنها أدفاؤها، ولا تقصوا أذنابها، فإنها مذابها.
Ahmad (17643) narrated it from Ali bin Bahr, who said, Baqiya bin Walid told us, Nasr bin Alqama told me, some men from Banu Salim told me, from Utba bin Abdul-Sulami, and he related it.This chain is hasan because of Nasr bin Alqama, who is Hadrami, from Homs. His hadith is hasan, a group of narrators transmitted from him, Duhaym affirmed his reliability, and Ibn Hibban mentioned him among the trustworthy. The men from Banu Salim are unnamed, but they may be accepted since they are a group of narrators. Baqiya bin Walid explicitly stated that he heard the hadith directly.Thawr bin Yazid also narrated it, from Nasr, from a man of Banu Salim, from Utba bin Abdul-Sulami, though there is considerable disagreement about the chain through Thawr. His version is found in Abu Dawud (2542) and Ahmad (17638, 17640) and elsewhere. However, this does not affect the first chain, since the two routes diverge at different points. And Allah knows best.As for his saying "their manes" (araafuha), this is the plural of urf, which refers to the hair on the neck of a horse. And his saying "their warmth" (adfaauha) is the plural of dif, meaning that which warms you, that is, it wards off the cold from you. And his saying "their fly-whisks" (midhabbuha) is the plural of midhabba, which is what is used to swat away flies.