He came to the Prophet ﷺ and accepted Islam on the condition that he would only pray two prayers, and the Prophet accepted that from him.
حدثنا محمد بن جعفر حدثنا شعبة عن قتادة عن نصر بن عاصم
أنه أتى النبي ﷺ فأسلم على أنه لا يصلي إلا صلاتين فقبل ذلك منه
Its isnad is sahih, and Nasr bin Aasim is among the trustworthy Followers, and it is established that he repented of the view of the Khawarij. On its face, the hadith as it stands seems unacceptable at first glance. But it has already been related that a Bedouin told the Prophet ﷺ that too many rulings had been placed upon him, so the Prophet advised him regarding prayer. He said, "I cannot manage it." The Prophet said, "Do not neglect the two cool prayers," meaning Asr and the dawn prayer. This was said for the sake of winning him over, and in one narration he said, "But he will indeed pray."