Musnad Ahmad 27:462

'Abd al-Razzaq narrated to us, a sheikh from the people of Najran informed us, Muhammad ibn 'Abd al-Rahman ibn al-Baylamani narrated to me, from his father, from Ibn 'Umar, that he asked the Prophet, may Allah bless him and grant him peace, or that a man asked the Prophet, may Allah bless him and grant him peace, and said: "What is permissible regarding witnesses in breastfeeding?" The Prophet, may Allah bless him and grant him peace, said: "A man and a woman."