Sahih Muslim | Book 19 | Chapter 3
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.
Narrated by Sa'd b. Abu Burda
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.
This hadith has been transmitted on the authority of Buraida but for the last two words.