Musannaf Ibn Abi Shayba 13:782

Abu Bakr narrated to us, he said: Ibn Abi Za'idah narrated to us, from Zakariya, from 'Amir, regarding a man who took a slave woman as a pledge, and she had a son whom she breastfed for him. He said: The equivalent wage for her breastfeeding is to be calculated for her.