Musannaf Ibn Abi Shayba 19:412

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Sa’id, from Qatada, from Jabir ibn Zayd—and it is the statement of Qatada—that they both said: "Separation is made between them by the testimony of two or three [witnesses], and [the punishment of] stoning is carried out by the testimony of four."