Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "People have never exchanged among themselves anything better than tarq (that is, lending one's male animal for mating). A man lends his horse for mating and its reward keeps flowing to him; a man lends his stallion camel for mating and its reward keeps flowing to him; and a man lends his ram for mating and its reward keeps flowing to him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13005