Musannaf Ibn Abi Shayba 13:1822

Abu Bakr narrated to us, he said: Abu Usamah narrated to us, from Sa’id ibn Abi ‘Arubah, from Qatadah, from al-Hasan, who said: Leave the People of the Scripture alone with their own judgments. But if they bring their cases to you, then establish upon them what is in your Book.