Sunan an-Nasā’ī Ḥadīth [5399] – Methodology in the Issuance of Fatāwā
It was narrated from Shurayḥ 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 Allāh. If it is not (mentioned) in the Book of Allāh, then (judge) according to the Sunnah of the Messenger of Allāh (peace and blessings of Allāh be upon him). If it is not (mentioned) in the Book of Allāh or the Sunnah of the Messenger of Allāh (peace and blessings of Allāh be upon him), then pass judgment according to the way the righteous passed judgment. If it is not (mentioned) in the Book of Allāh, or the Sunnah of the Messenger of Allāh (peace and blessings of Allāh 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.”