Surah as Saffat verse 35:
إِنَّهُمْ كَانُوا إِذَا قِيلَ لَهُمْ لَا إِلَٰهَ إِلَّا اللَّهُ
"For when it was said unto them, There is no Allah save Allah, they were scornful"
One of the first people to embrace Islam was Sayyiduna Abu Bakr as Siddeeq may Allah be pleased with him. The account of his conversion to Islam is as follows: That he returned from a business trip to Yemen and found the people gossiping about Muhammad peace be upon him, that he was claiming to be a prophet. Upon hearing this Abu Bakr came to his house and asked him. When Muhammad explained to him that he was a prophet sent by God, Abu Bakr finally said:
مد يدك، فأنا اشهد أن لا إله إلا الله، وأنك رسول الله.
("Stretch out your hand, for I testify that there is no god but God and you are His messenger")
Source: taken from the book al Asad al Ghaba
And Allah knows best