It was narrated from Shuraih that: He wrote to ‘Umar (RA,) 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
(peace be upon him). If it is not (mentioned) in the Book of Allah or
the Sunnah of the Messenger of Allah (peace be upon him), 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 (peace be upon him), 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.”

(Sunan an-Nasa’I, Vol. 6, Book 49, Hadith 21)