Adam Clarke

By a river side, where prayer was wont to be made - Οὑ ενομιζετο προσευχη ειναι, where it was said there was a proseucha. The proseucha was a place of prayer, or a place used for worship, where there was no synagogue. It was a large building uncovered, with seats, as in an amphitheatre. Buildings of this sort the Jews had by the sea side, and by the sides of rivers. See this subject considered at large in the note on Luk 6:12 (note). It appears that the apostles had heard from some of the Gentiles, or from some of the Jews themselves, that there was a place of prayer by the river side; and they went out in quest of it, knowing that, as it was the Sabbath, they should find some Jews there. Spake unto the women - Probably this was before the time of their public worship, and while they were waiting for the assembling of the people in general; and Paul improved the opportunity to speak concerning Christ and salvation to the women that resorted thither.