On the day of Khaybar we slaughtered horses, mules, and assess. The Messenger of Allah ﷺ forbade us (to eat) mules and asses, but he did not forbid horse-flesh. (Using translation from Abū Dāʾūd 3789)
ذَبَحْنَا يَوْمَ خَيْبَرَ الْخَيْلَ وَالْبِغَالَ وَالْحَمِيرَ فَنَهَانَا رَسُولُ اللهِ ﷺ عَنِ الْبِغَالِ وَالْحَمِيرِ وَلَمْ يَنْهَنَا عَنِ الْخَيْلِ