Sahih Muslim 1732
Sahih Muslim · Book 32: The Book of Jihad and Expeditions · The command to show leniency and avoid causing aversion (towards Islam)
It has been narrated on the authority of Abu Masa that when the Messenger of Allah (ﷺ) deputed any of his Companions on a mission, he would say:
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، وَأَبُو كُرَيْبٍ - وَاللَّفْظُ لأَبِي بَكْرٍ - قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ عَنْ بُرَيْدِ بْنِ عَبْدِ اللَّهِ ، عَنْ أَبِي بُرْدَةَ ، عَنْ أَبِي مُوسَى ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا بَعَثَ أَحَدًا مِنْ أَصْحَابِهِ فِي بَعْضِ أَمْرِهِ قَالَ " بَشِّرُوا وَلاَ تُنَفِّرُوا وَيَسِّرُوا وَلاَ تُعَسِّرُوا " .
Give tidings (to the people) ; do not create (in their minds) aversion (towards religion) ; show them leniency and do not be hard upon them.