Musannaf Ibn Abi Shayba 7:292

Abu al-Ahwas narrated to us, from Abu Ishaq, who said: A slave girl threw a bundle of reeds onto a young boy, and it killed him. So, the matter was brought to Masruq. He said: "Do you know of any masters for her?" They said: "We do not know who her masters are." He said: "Does she have any wealth?" They said: "We do not know of any wealth." He said: "Then command her to fast for two consecutive months."