Musnad Abu Ya'la al-Mawsili 115:105

Ubayd Allah ibn Mu'adh narrated to us, Mu'adh ibn Hisham narrated to us, my father narrated to me, from Qatadah, that Anas ibn Malik narrated to him, saying: When the Prophet of Allah, Musa, peace be upon him, called his companion to the term that was between them, his companion said to him: "Every sheep that gives birth to [a lamb] of a color other than its own, its offspring is yours." He said: So he proceeded and placed ropes upon the water. When they [the sheep] saw the ropes, they became frightened and moved about in a state of agitation, so they all gave birth to spotted [offspring], except for one sheep. He said: So he took their offspring that year.