According to Luke, in Acts 9:15 he did:

"But the Lord said to him [Ananias], “Go, for he [Paul] is a chosen instrument of mine to carry my name before the Gentiles and kings and the children of Israel."

Apart from that, Peter also understood that Paul had been authenticated by Jesus as his apostle, see 2 Peter 3:

"Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction."

Being an apostle meant that Paul was expressly authorized and enabled to represent the Lord. Hence the saying, "you can't slip a piece of paper..." meaning that one can't try and play one off against the other, as if they hold opposing views. Paul represents Jesus, nothing more, nothing less. Paul is merely a servant, the words are those of Jesus.