Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4298
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.