Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: When this verse was revealed: "O Prophet, indeed We have sent you as a witness, a bearer of glad tidings and a warner" [Surah al-Ahzab: 45], the Prophet (peace and blessings be upon him) called Sayyiduna Ali and Sayyiduna Mu'adh (may Allah be pleased with them both), whom he had ordered to go to Yemen, and said: "Go, and give glad tidings and do not drive people away; make things easy and do not make them difficult, for this verse has been revealed to me: O Prophet, indeed We have made you a witness, that is, over your Ummah; and a bearer of glad tidings, that is, of Paradise; and a warner, that is, of the Fire; and one who invites, that is, to the testimony that there is no true god except Allah; and a shining lamp, that is, through the Qur'an."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11791