Qais bin Saʿd was to the Prophet ﷺ like a chief police officer to an Amir (chief).  

أَنَّ قَيْسَ بْنَ سَعْدٍ كَانَ يَكُونُ بَيْنَ يَدَىِ النَّبِيِّ ﷺ بِمَنْزِلَةِ صَاحِبِ الشُّرَطِ مِنَ الأَمِيرِ  

that the Prophet ﷺ sent him and sent Muʿadh after him (as rulers to Yemen).  

أَنَّ النَّبِيَّ ﷺ بَعَثَهُ وَأَتْبَعَهُ بِمُعَاذٍ  

A man embraced Islam and then reverted back to Judaism. Muʿadh bin Jabal came and saw the man with Abu Musa. Muʿadh asked, "What is wrong with this (man)?" Abu Musa replied, "He embraced Islam and then reverted back to Judaism." Muʿadh said, "I will not sit down unless you kill him (as it is) the verdict of Allah and His Apostle.  

لاَ أَجْلِسُ حَتَّى أَقْتُلَهُ قَضَاءُ اللَّهِ وَرَسُولِهِ ﷺ