The Messenger of Allah ﷺ said: Tie the horses, rub down their forelocks and their buttocks (or he said: Their rumps), and put things on their necks, but do not put bowstrings. (Using translation from Abū Dāʾūd 2553)   

قَالَ رَسُولُ اللهِ ﷺ ارْتَبِطُوا الْخَيْلَ وَامْسَحُوا بِنَوَاصِيهَا وَأَعْجَازِهَا أَوْ قَالَ وَأَكْفَالِهَا وَلَا تُقَلِّدُوهَا الْأَوْتَارَ