Musannaf Ibn Abi Shayba 18:280

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Ibn Abi Najih, from Mujahid, who said regarding the thumb: "Fifteen [camels are due]." And regarding the one that follows it: "Ten." And regarding the one that follows it: "Eight." And regarding the one that follows it: "Seven."