Musannaf Ibn Abi Shayba 9:1135

Abu Khalid narrated to us, from Hajjaj, from Abu al-Zubayr, from Ibn Tawus, from his father, and Humayd al-A'raj, from Mujahid, from whoever heard 'Ata' say: Zam'ah said, and I heard 'Amr ibn Dinar say: They said, "A small amount of breastfeeding and a large amount of it makes [marriage] prohibited."