Sahih Muslim Book 19, Hadith Number 4297.

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

It has been narrated on the authority of Abu Masa that when the Messenger of Allah (may peace be upon him) deputed any of his Companions on a mission, he would say: Give tidings (to the people); do not create (in their minds) aversion (towards religion); show them leniency and do not be hard upon them.