Ali Ibn Abi Talib was sent to preach the message of Islam to the people of Hamdan. Inviting them to embrace Islam if they were convinced with the Message.
“The Messenger of God sent Khalid b. al-Walid to the people of the Yemen inviting them to Islam, and I was among those who went with him. He persisted in the matter for six months, but they did not respond, so the Messenger of God sent Ali b. Abi Talib and ordered him that Khalid and those who were with him should return, but if any of them would like to follow him he should allow them. Al-Bara said, ‘I was one who followed Ali, and as we reached the borders of the Yemen the people got the news. They gathered around him and Ali led us in the Morning Prayer. When he had finished [the prayer], he lined us up in one row. Then he moved before us, praised and extolled God, and then read to them the letter of the Messenger of God. All of Hamdan embraced Islam in one day, and he wrote to the Messenger of God about it. When the Prophet read Ali’s letter he fell down, prostrating himself to God. Then he sat up and said, ‘Peace be upon Hamdan, peace be upon Hamdan!’ [After the conversion of Hamdan] the people of the Yemen followed in succession with their acceptance of Islam.’” 
The arrival of this delegation was in the ninth year of Al-Hijra after the Messenger’s return from Tabuk. So the Messenger of Allah (p) gave them a pledge to guarantee the fulfilment of their demands. He sent to them Malik bin An Namt as their chief, and appointed him as an agent on those of his people who embraced Islam. Khalid bin Al-Waleed was appointed for them all with a purpose of calling them to Islam. HE STAYED WITH THEM FOR SIX MONTHS CALLING TO ISLAM BUT NO ONE RESPONDED TO HIS CALL. Later on Ali bin Abi Talib was dispatched there and Khalid was ordered to come back. UPON ARRIVING AT HAMDAN, ALI COMMUNICATED TO THEM A MESSAGE FROM THE MESSENGER OF ALLAH (P) AND CALLED THEM TO ISLAM. They responded to the call and became Muslims. Ali wrote to the Messenger of Allah (p) of the good tidings. The Messenger of Allah (p) was briefed on the content of that letter, so he prostrated, then raised his head up and said: ‘peace be upon you Hamdan. Peace be upon you Hamdan.’” 
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 History of al-Tabari: The: The Last Years of the Prophet, volume 9, page 90
 Sealed Nectar (‘Ar-Raheeq Al-Makhtum’), By Safiur-Rahman Al-Mubarakpuri, page 202