Sunan an-Nasa'i 5399
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 [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."
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا أَبُو عَامِرٍ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الشَّيْبَانِيِّ، عَنِ الشَّعْبِيِّ، عَنْ شُرَيْحٍ، أَنَّهُ كَتَبَ إِلَى عُمَرَ يَسْأَلُهُ فَكَتَبَ إِلَيْهِ أَنِ اقْضِ بِمَا فِي كِتَابِ اللَّهِ فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللَّهِ فَبِسُنَّةِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللَّهِ وَلاَ فِي سُنَّةِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَاقْضِ بِمَا قَضَى بِهِ الصَّالِحُونَ فَإِنْ لَمْ يَكُنْ فِي كِتَابِ اللَّهِ وَلاَ فِي سُنَّةِ رَسُولِ اللَّهِ صلى الله عليه وسلم وَلَمْ يَقْضِ بِهِ الصَّالِحُونَ فَإِنْ شِئْتَ فَتَقَدَّمْ وَإِنْ شِئْتَ فَتَأَخَّرْ وَلاَ أَرَى التَّأَخُّرَ إِلاَّ خَيْرًا لَكَ وَالسَّلاَمُ عَلَيْكُمْ‏.‏