Musannaf Ibn Abi Shayba 18:418

Abu Bakr narrated to us, he said: Yazid bin Harun narrated to us, from Hajjaj, from Makhul, from Zayd bin Thabit; and from Hajjaj, from Abu Ishaq; and from Hajjaj, from al-Zuhri, from Urwah bin al-Zubayr; and from Hajjaj, from Amr bin Shu’ayb, that they said: The two testicles are equal.