It is narrated from al-Hasan (may Allah have mercy on him) that a man sold a slave girl belonging to his father while his father was absent. When the father came, he refused to let the sale stand, and the slave girl had borne a child to the buyer. They brought their dispute to Sayyiduna Umar (may Allah be pleased with him). He ruled that the slave girl belonged to her owner, and ordered the buyer to take back his price, and so he did. The seller's father said: "Tell him to let go of my son." Sayyiduna Umar (may Allah be pleased with him) said to him: "And you let go of his son."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الغصب / 11547