Sunan at-Tirmithiy كتاب البيوع باب ما جاء فى كراهية عسب الفحل
A man from Kilāb asked the Prophet, peace and blessings of God be upon him, about the hiring out of a bull for breeding, so he forbade him (from asking for a sum of money). The man retoerted, "Messenger of God, we lend our bulls for breeding and are rewarded." So he permitted the acceptance of the gift. Click on ḥadīth to read more