Musannaf Ibn Abi Shayba 35:128

Abu Usama narrated to us, from Yahya ibn Sa'id, who said: Sa'id ibn al-Musayyib said: "Whoever emancipates his slave girl or the mother of his child and makes that her dowry, I consider that permissible for him." And he mentioned that Abu Hanifa said: "It is not permissible except with a [separate] dowry."