At Pentecost, Peter said the outpouring was what had been spoken by which prophet?

Answer: Joel

Peter said, "This is what was spoken by the prophet Joel: 'In the last days... I will pour out My Spirit on all people.'"

“No, this is what was spoken by the prophet Joel: ‘In the last days, God says, I will pour out My Spirit on all people. Your sons and daughters will prophesy, your young men will see visions, your old men will dream dreams.”— Acts 2:16-17 (BSB)