كتاب الجهاد والسير
32
The Book of Jihad and Expeditions
(3)
Chapter: The command to show leniency and avoid causing aversion (towards Islam)
(3)
باب فِي الأَمْرِ بِالتَّيْسِيرِ وَتَرْكِ التَّنْفِيرِ

It has also been narrated by Sa'd b. Abu Burda through his father through his grandfather that the Prophet of Allah (ﷺ) sent him and Mu'adh (on a mission) to the Yemen, and said (by way of advising them):

Show leniency (to the people) ; don't be hard upon them; give them glad tidings (of Divine favours in this world and the Hereafter) ; and do not create aversion. Work in collaboration and don't be divided.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ شُعْبَةَ، عَنْ سَعِيدِ بْنِ أَبِي بُرْدَةَ، عَنْ أَبِيهِ، عَنْ جَدِّهِ، أَنَّ النَّبِيَّ صلى الله عليه وسلم بَعَثَهُ وَمُعَاذًا إِلَى الْيَمَنِ فَقَالَ ‏ "‏ يَسِّرَا وَلاَ تُعَسِّرَا وَبَشِّرَا وَلاَ تُنَفِّرَا وَتَطَاوَعَا وَلاَ تَخْتَلِفَا ‏"‏ ‏.‏
Reference : Sahih Muslim 1733 a
In-book reference : Book 32, Hadith 7
USC-MSA web (English) reference : Book 19, Hadith 4298
  (deprecated numbering scheme)