Ali ibn Abu Taalib (a Companion) narrated
that the Prophet of Allah, sallallaahu ‘alayhi wa sallam, was presented with a she-mule which he rode, so Ali said: “If we made asses cover mares we would have animals of this type.” The Prophet of Allah, sallallaahu ‘alayhi wa sallam, said: “Only those who do not know do that.”1
It was said that the reason this is not recommended is that it would be exchanging something good (horse breed) with that which is less (mules).
Al-Khattaabi (an exegete of Prophetic narrations) said: “The meaning of this is that if asses cover mares, then horses would decrease in number and could even become extinct, and thus they would no longer exist, and people would lose their benefit. Horses are needed to ride, seek provisions, perform Jihaad, gain war booty, in addition to their meat being edible, while none of this is the case with asses. Therefore, the Prophet of Allah, sallallaahu ‘alayhi wa sallam, wanted to increase the number of horses so that people would benefit from them.”2