Musannaf Ibn Abi Shayba 26:505

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Ibrahim—Al-A’mash said: I think it is from Al-Aswad—he said: Umar said: If a free woman is under a slave and she gives birth, then the *wala’* (patronage) of her child belongs to the masters of the mother. Then, if the father is emancipated, he draws the *wala’* (to himself).