Musannaf Ibn Abi Shayba 13:2687

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, from Abd al-Malik, from Ata, who said: Do not take a fee for the stud’s covering. And there is no harm if you give [a fee] if you do not know [of another way] or you do not find anyone to let your animal be covered by his.