He became a Muslim at the hands of Musab ibn Umayr (ra) whose cool and persuasive logic and the sweetness and beauty of his Quran recital proved irresistible.
When the Prophet (saw) arrived in Madinah after the historic Hijrah (emigration), Thabit and a great gathering of horsemen gave him a warm and enthusiastic welcome. Thabit acted as their spokesman and delivered a speech in the presence of the Prophet and his companion, Abu Bakr as-Siddiq (ra). He began by giving praise to God Almighty and invoking peace and blessings on His Prophet and ended up by saying:
"We give our pledge to you, O Messenger of God (saw), that we would protect you from all that we protect ourselves, our children and our wives. What would then be our reward for this?"
The speech was reminiscent of words spoken at the second Pledge of Aqabah and the Prophet's (saw) reply as then was the same: "Al-Jannah - Paradise!"
When the Yathribites heard the word "al-Jannah" their faces beamed with happiness and excitement and their response was: "We are pleased, O Messenger of God! We are pleased, O Messenger of God (saw)."
From that day on the Prophet, peace be on him, made Thabit ibn Qays (ra) his Khatib, just as Hassan ibn Thabit (ra) was his poet. When delegations of Arabs came to him to show off their brilliance in verse and the strength of their oratory skills which the Arabs took great pride in, the Prophet (saw) would call upon Thabit ibn Qays (ra) to challenge their orators and Hassan ibn Thabit (ra) to vaunt his verses before their poets.
In the Year of the Delegations, the ninth after the Hijrah, tribes from all over the Arabian peninsula came to Madinah to pay homage to the Prophet (saw), either to announce their acceptance of Islam or to pay jizyah in return for the protection of the Muslim state. One of these was a delegation from the tribe of Tamim who said to the Prophet (saw):
"We have come to show our prowess to you. Do give permission to our Shaif and our Khatib to speak." The Prophet, peace be on him, smiled and said: "I permit your Khatib. Let him speak."
Their orator, Utarid ibn Hajib, got up and held forth on the greatness and achievements of their tribe and when he was finished the Prophet (saw) summoned Thabit ibn Qays (ra) and said: "Stand and reply to him." Thabit (ra) arose and said:
"Praise be to God Whose creation is the entire heavens and the earth wherein His will has been made manifest. His Throne is the extent of His knowledge and there is nothing which does not exist through His grace.
"Through His power He has made us leaders and from the best of His creation He has chosen a Messenger who is the most honorable of men in lineage, the most reliable and true in speech and the most excellent in deeds. He has revealed to him a book and chosen him as a leader of His creation. Among all creation, he is a blessing of God.
"He summoned people to have faith in Him. The Emigrants from among his people and his relations who are the most honorable people in esteem and the best in deeds believed in him. Then, we the Ansar (Helpers) were the first people to respond (to his call for support). So we are the Helpers of God and the ministers of His Messenger."
Thabit (ra) was a believer with a profound faith in God. His consciousness and fear of God was true and strong. He was especially sensitive and cautious of saying or doing anything that would incur the wrath of God Almighty. One day the Prophet (saw) saw him looking not just sad but dejected and afraid. His shoulders were haunched and he was actually cringing from fear.
"What's wrong with you, O Abu Muhammad?" asked the Prophet (saw). "I fear that I might be destroyed, O Messenger of God (saw)," he said. "And why?" asked the Prophet (saw). "God Almighty," he said, "has prohibited us from desiring to be praised for what we did not do but I find myself liking praise. He has prohibited us from being proud and I find myself tending towards vanity." This was the time when the verse of the Quran was revealed: "Indeed, God does not love any arrogant boaster."
The Prophet, peace be on him, then tried to calm his anxieties and allay his fears and eventually said to him: "O Thabit (ra), aren't you pleased to live as someone who is praised, and to die as a martyr and to enter Paradise?"
Thabit's (ra) face beamed with happiness and joy as he said: "Certainly, O Messenger of God (saw)." "Indeed, that shall be yours," replied the noble Prophet (saw).
There was another occasion when Thabit (ra) became sad and crest-fallen, when the words of the Quran were revealed:
"O you who believe! Raise not your voices above the voice of the Prophet and neither speak loudly to him as you would speak loudly to one another, lest all your deeds come to naught without your perceiving it."
On hearing these words, Qays (ra) kept away from the meetings and gatherings of the Prophet (saw) in spite of his great love for him and his hitherto constant presence in his company. He stayed in his house almost without ever leaving it except for the performance of the obligatory Salat (prayer). The Prophet (saw) missed his presence and evidently asked for information about him. A man from the Ansar volunteered and went to Thabit's (ra) house. He found Thabit (ra) sitting in his house, sad and dejected, with his head bowed low.
"What's the matter with you?" asked the man. "It's bad," replied Thabit (ra). "You know that I am a man with a loud voice and that my voice is far louder than that of the Messenger of God, may God bless him and grant him peace. And you know what has been revealed in the Quran. The only result for me is that my deeds will come to naught and I will be among the people who go to the fire of hell."
The man returned to the Prophet (saw) and told him what he had seen and heard and the Prophet (saw) instructed him to return to Thabit (ra) and say: "You are not among the people who will go to the fire of hell but you will be among the people of Paradise."
Such was the tremendously good news with which Thabit ibn Qays (ra) was blessed. The incidents showed how alive and sensitive he was to the Prophet (saw) and the commands of Islam and his readiness to observe the letter and the spirit of its laws. He subjected himself to the most stringent self-criticism. His was a God-fearing and penitent heart which trembled and shook through the fear of God.