ENOfficial
‘Utba b. ‘Abd as-Sulami told that he heard God’s Messenger say, “Do not cut the forelocks, manes, or tails of horses, for their tails are their means of driving off flies, their manes provide them with warmth, and blessing is tied to their forelocks.” Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَن عُتبةَ بن عبدٍ السُّلميِّ أَنَّهُ سَمِعَ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «لَا تَقُصُّوا نَوَاصِيَ الْخَيْلِ وَلَا مَعَارِفَهَا وَلَا أَذْنَابَهَا فَإِنَّ أَذْنَابَهَا مَذَابُّهَا وَمَعَارِفَهَا دِفاءُها وَنَوَاصِيهَا مَعْقُودٌ فِيهَا الْخَيْرُ» . رَوَاهُ أَبُو دَاوُدَ
sunnah.com