Musnad Ahmad 27:463

Ibn Abi Shaybah narrated to us, from Mu'tamir, from Muhammad ibn 'Uthaym, from Muhammad ibn 'Abd al-Rahman—meaning this hadith. Abu 'Abd al-Rahman 'Abdullah ibn Ahmad said: And Abu Bakr 'Abdullah ibn Abi Shaybah narrated to us, he said: 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 Messenger of Allah, peace and blessings of Allah be upon him, was asked: "What is permissible regarding witnesses in breastfeeding?" He said: "A man and a woman."