When the Messenger of Allah ﷺ intended to send Mu'adh ibn Jabal to the Yemen, he asked: How will you judge when the occasion of deciding a case arises? He replied: I shall judge in accordance with Allah's Book. He asked: (What will you do) if you do not find any guidance in Allah's Book? He replied: (I shall act) in accordance with the Sunnah of the Messenger of Allah ﷺ. He asked: (What will you do) if you do not find any guidance in the Sunnah of the Messenger of Allah ﷺ and in Allah's Book? He replied: I shall do my best to form an opinion and I shall spare no effort. The Messenger of Allah ﷺ then patted him on the breast and said: Praise be to Allah Who has helped the messenger of the Messenger of Allah to find something which pleases the Messenger of Allah.  

أَنَّ رَسُولَ اللَّهِ ﷺ لَمَّا أَرَادَ أَنْ يَبْعَثَ مُعَاذًا إِلَى الْيَمَنِ قَالَ كَيْفَ تَقْضِي إِذَا عَرَضَ لَكَ قَضَاءٌ قَالَ أَقْضِي بِكِتَابِ اللَّهِ قَالَ فَإِنْ لَمْ تَجِدْ فِي كِتَابِ اللَّهِ قَالَ فَبِسُنَّةِ رَسُولِ اللَّهِ ﷺ قَالَ فَإِنْ لَمْ تَجِدْ فِي سُنَّةِ رَسُولِ اللَّهِ ﷺ وَلاَ فِي كِتَابِ اللَّهِ قَالَ أَجْتَهِدُ رَأْيِي وَلاَ آلُو فَضَرَبَ رَسُولُ اللَّهِ ﷺ صَدْرَهُ وَقَالَ الْحَمْدُ لِلَّهِ الَّذِي وَفَّقَ رَسُولَ رَسُولِ اللَّهِ لِمَا يُرْضِي رَسُولَ اللَّهِ  

Mu'adh bin Jabal said that when the Messenger of Allah ﷺ sent him to the Yemen... He then narrated the rest of the tradition to the same effect.  

أَنَّ رَسُولَ اللَّهِ ﷺ لَمَّا بَعَثَهُ إِلَى الْيَمَنِ فَذَكَرَ مَعْنَاهُ