John Calvin Commentary Acts 9:11

"And the Lord [said] unto him, Arise, and go to the street which is called Straight, and inquire in the house of Judas for one named Saul, a man of Tarsus: for behold, he prayeth;" — Acts 9:11 (ASV)

For, behold, he prayeth. Luke shows that Paul devoted himself to prayer during those three days. Perhaps this was one reason why he fasted; although it is certain, as I have already said, that he endured such prolonged hunger because he was, in a way, deprived of his senses, as people who are in a trance usually are.

Christ certainly does not speak of a short prayer in this place; rather, He shows that Paul continued in this type of exercise until he could be more quiet in mind.

For besides other causes of terror, that voice might have sounded in his ears, Saul, Saul, why persecutest thou me? And it is not to be doubted that the anxious expectation of a perfect revelation marvelously troubled his mind; but this was the reason the Lord caused him to wait three days, so that He might more effectively kindle in him an earnest desire to pray.