Musannaf Ibn Abi Shayba 35:91

Mu'tamir narrated to us, from Muhammad ibn 'Uthaym, from Muhammad ibn 'Abd al-Rahman ibn al-Baylamani, from his father, from Ibn 'Umar, who said: The Prophet, peace and blessings of Allah be upon him, was asked: "What is permissible regarding witnesses in [matters of] breastfeeding?" He said: "A man or a woman." And he mentioned that Abu Hanifa said: "It is not permissible except with more [than that]."