Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4297
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.