I know you want this proof, but you really will not get it in such a way that answers a person who wants a verse saying, “Mary had no sin, either actual or original”.
If Jesus said it and it was written in the Gospels, they think they would believe it.
But Jesus did say it in the Gospels: “If anyone hears you (apostles) they hear me”. Peter did tell the next Pope, who told the next, who told the next,… who told John Paul II, who told Benedict, who told Francis, who told your Bishop, who told your Pastor, who told you, and you “hear and believe”, so you have heard and believe Jesus.
I am one of you, now, a Catholic, because I came to recognize that apostolic succession and authority is real. After understanding and believing that authority, I came to the teachings about Mary, and believe them because I believe the apostles that are hear today teaching me all that Jesus commanded, all things that the sent Holy Spirit revealed to them. Before trusting today’s living apostles were really from Jesus, I did not believe Mary was “sinless” or “immaculate”, etc.