Musannaf Ibn Abi Shayba 9:1130

Ibn Fudayl narrated to us, from Layth, from Mujahid, who said: Ibn Mas’ud said, "A small amount of breastfeeding prohibits [marriage] just as a large amount of it prohibits [it]." And Mujahid said, "The statement of Ibn Mas’ud is more beloved to me."