Abu Dawud
Jihad
Book Number: 008 / Hadith Number: 2559
Narated By Ali ibn AbuTalib : The Apostle of Allah (pbuh) was present with a she-mule which he rode, so Ali said: If we made asses cover mares we would have animals of this type. The Apostle of Allah (pbuh) said: Only those who do not know do that.