Musannaf Ibn Abi Shayba 7:291

Yazid ibn Harun narrated to us, he said: Dawud ibn Abi Hind informed us, from al-Sha’bi, who said: A caravan from the people of Syria passed by and bought a slave girl, then they emancipated her. She threw a bundle of reeds at a young boy and killed him. She was brought to Masruq, and he said: "Seek out her guardians," but they did not find anyone. He looked for a while and remembered, and said: "Allah said: 'And whoever does not find [the means], then a fast of two months consecutively.' Go, and fast for two months consecutively, and they have no claim against you."