Musnad Ahmad 71:0

Narrated to us by Yahya ibn Sa’id, from Hisham, who said: My father narrated to me, from Al-Arqam ibn Abi Al-Arqam, that he said: The Messenger of Allah (ﷺ) used to pray in the house of Al-Arqam. He remained there until he had gathered forty men, the last of whom was ‘Umar ibn Al-Khattab. When they reached forty men, they emerged and proclaimed the religion openly. So, Allah, the Mighty and Majestic, revealed: "O Prophet, sufficient for you is Allah and whoever follows you of the believers."