Abu Dawud Book 8, Hadith Number 2559.
Chapter : Not known.
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.