Musannaf Ibn Abi Shayba 18:423

Abu Bakr narrated to us, he said: Isma'il bin Ibrahim narrated to us, from Dawud, from Sa'id bin al-Musayyib, who said regarding the left testicle: "Two-thirds of the blood money," and regarding the right one: "One-third." I said: "Why?" He said: "Because if the left one is gone, he will not have children, and if the right one is gone, he will have children."