Sahih Muslim Book 19, Hadith Number 4298.

Chapter : Command to show leniency and to avoid creating aversion (towards religion).

It has also been narrated by Sa’d b. Abu Burda through his father through his grandfather that the Prophet of Allah (may peace be upon him) 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.