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 [SAW]. If it is not (mentioned) in the Book of Allah or the Sunnah of the Messenger of Allah [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 [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." (Using translation from Nasāʾī 5399)  

«أَنِ اقْضِ بِمَا فِي كِتَابِ اللهِ فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللهِ فَبِسُنَّةِ رَسُولِ اللهِ ﷺ فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللهِ وَلَا سُنَّةِ رَسُولِ اللهِ ﷺ فَاقْضِ بِمَا قَضَى بِهِ الصَّالِحُونَ فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللهِ وَلَا فِي سُنَّةِ رَسُولِ اللهِ ﷺ وَلَمْ يَقْضِ بِهِ الصَّالِحُونَ فَإِنْ شِئْتَ فَتَقَدَّمْ وَإِنْ شِئْتَ فَتَأَخَّرْ وَلَا أَرَى التَّأَخُّرَ إِلَّا خَيْرًا لَكَ وَالسَّلَامُ عَلَيْكُمْ»