Musannaf Ibn Abi Shayba 9:523

Abu Bakr narrated to us, he said: Mu'tamir narrated to us, from Muhammad ibn Tamim, from Muhammad ibn Abd al-Rahman al-Salman, from his father, from Ibn Umar, who said: The Prophet of Allah was asked what is permissible regarding witnesses in breastfeeding. He said: "A man and a woman."