Musannaf Ibn Abi Shayba 13:3031

Abu Bakr narrated to us, he said: Ali bin Mushir narrated to us, from al-Shaybani, from al-Sha’bi, from Shurayh, that Umar bin al-Khattab, may Allah be pleased with him, wrote to him: If something comes to you in the Book of Allah, judge by it and do not let men turn you away from it. If a matter comes to you that is not in the Book of Allah, then look to the Sunnah of the Messenger of Allah, may Allah bless him and grant him peace, and judge by it. If something comes to you that is not in the Book of Allah and there is no Sunnah in it from the Messenger of Allah, may Allah bless him and grant him peace, then look at what the people have agreed upon and take it. If something comes to you that is not in the Book of Allah, and there is no Sunnah in it from the Messenger of Allah, may Allah bless him and grant him peace, and no one has spoken about it before you, then choose whichever of the two matters you wish. If you wish to exert your opinion and proceed, then proceed; and if you wish to hold back, then hold back. And I do not see holding back as anything but better for you.