سنن النسائي

It was narrated from Shuraih that: He wrote to 'Umar, to ask him (a question), and 'Umar wrote back to him telling him: "Judge according to what is in the Book of Allah. If it is not (mentioned) in the Book of Allah, then (judge) according to the Sunnah of the Messenger of Allah (ﷺ) [SAW]. If it is not (mentioned) in the Book of Allah or the Sunnah of the Messenger of Allah (ﷺ) [SAW], then pass judgment according to the way the righteous passed judgment. If it is not (mentioned) in the Book of Allah, or the Sunnah of the Messenger of Allah (ﷺ) [SAW], and the righteous did not pass judgment concerning it, then if you wish, go ahead (and try to work it out by yourself) or if you wish, leave it. And I think that leaving it is better for you. And peace be upon you."