He (PBUH) gave them the glad tidings of reward for the good deeds they used to do before Islam

‘ narrated that Hakeem ibn Hizaam (another Companion) freed one hundred slaves before Islam and donated one hundred camels. When he became Muslim, he donated one hundred camels and freed one hundred slaves. He said: “I asked the Prophet of Allah, sallallaahu ‘alayhi wa sallam, ‘O Messenger of Allah, what do you think about some good deeds I had done before Islam, regarding them as deeds of righteousness?’” The Prophet of Allah, sallallaahu ‘alayhi wa sallam, said: “You have become Muslim with the good deeds you did previously.