Musnad Ahmad 937:222

Ubayd Allah ibn Muhammad narrated to us, Hammad ibn Salamah informed us, Layth informed us, from Abd al-Rahman ibn Tharwan, from al-Huzayl ibn Shurahbil, from Abu Dharr, that the Messenger of Allah (may Allah bless him and grant him peace) was sitting, and two sheep were eating fodder, and one of them butted the other, and it caused it to miscarry. He said: The Messenger of Allah (may Allah bless him and grant him peace) laughed. It was said to him: "What makes you laugh, O Messenger of Allah?" He said: "I am amazed at it. By the One in Whose hand is my soul, it shall surely be retaliated for on the Day of Resurrection."