Musannaf Ibn Abi Shayba 36:70

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, he said: Abd al-Malik narrated to us, he said: Abu Fazara narrated to us, saying: The Prophet, peace and blessings of Allah be upon him, saw Zayd ibn Harithah as a boy with a braided lock of hair, whom his people had stationed at al-Batha' to sell him. He came to Khadijah and said: "I saw a boy at al-Batha' whom they have stationed to sell him, and if I had his price, I would have bought him." She said: "And how much is his price?" He said: "Seven hundred." She said: "Take seven hundred and go and buy him." So he bought him and brought him to her. He said: "As for me, if he were mine, I would have freed him." She said: "Then he is yours," so he freed him.