Musnad Ahmed: Hadith 1976

The Messenger of Allah (ﷺ) was an obedient slave; he conveyed - by Allah - what he was sent with and he did not tell us anything in exclusion to the people, except for three things: He instructed us to do wudoo’ properly, not to consume charity and not to breed a donkey with a mare. Moosa said: I met "Abdullah bin Hasan and said: "Abdullah bin "Ubaidullah told me such and such. He said: Horses were few among Banu Hashim, and he wanted to increase their numbers.