Musannaf Ibn Abi Shayba 13:3119

Abu Bakr narrated to us, he said: Yahya ibn Adam narrated to us, from Hasan ibn Salih, he said: I said to al-Ja'd ibn Dhakwan: "Did you witness Shurayh saying: 'I permit the testimony of a witness against a witness if he testifies against both of them'?"