Musannaf Ibn Abi Shayba 13:3035

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Ubayd Allah ibn Abi Yazid, he said: When Ibn ‘Abbas was asked about a matter, if it was in the Qur’an, he would inform of it. And if it was not in the Qur’an, he would inform of it from the Messenger of Allah, may Allah bless him and grant him peace. And if it was not [there], then from Abu Bakr and ‘Umar, may Allah be pleased with them both. And if it was not [there], he would state his opinion regarding it.