John Gill Commentary Acts 22:15

"For thou shalt be a witness for him unto all men of what thou hast seen and heard." — Acts 22:15 (ASV)

For you shall be his witness to all men
Gentiles as well as Jews, an eye and an ear witness to them;

of what you have seen and heard ;
as that he saw him personally and alive, and so could witness to the truth of his resurrection; for after he had been seen by all the apostles, he was last of all seen of Paul; and also, that he heard him and received from him the Gospel, and a mission and commission to preach it; for what he preached he did not receive of man, nor was he taught it by any, but he had it by revelation from Jesus Christ.