Hadith: We slaughtered a horse and ate it at the time of the Messenger of Allah (may Allah's peace and blessings be upon him).

عن أَسْمَاء بِنْت أَبِي بَكْرٍ رضي الله عنهما قالت: «نَحَرْنَا عَلَى عَهْدِ رَسُولِ الله صلى الله عليه وسلم فَرَسًا فَأَكَلْنَاهُ». وَفِي رِوَايَةٍ «وَنَحْنُ بِالْمَدِينَةِ».
[صحيح] - [متفق عليه]

Asmā’ bint Abu Bakr (may Allah be pleased with her) reported: We slaughtered a horse and ate it at the time of the Messenger of Allah (may Allah's peace and blessings be upon him. Another narration has the following addition: "while we were in Madīnah."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Asmā’ bint Abu Bakr (may Allah be pleased with her) reports that during the Prophet's lifetime, they slaughtered a horse and ate it. This proves the lawfulness of eating horse meat. Therefore, no one should mistakenly believe that horse meat is unlawful to eat just because horses are mentioned along with donkeys and mules in the verse where Allah, the Exalted, says: {And the horses, mules, and donkeys are for you to ride, and as adornment. And He creates what you do not know.} [An-Nahl: 8]