Albert Barnes Commentary Acts 2:16

"but this is that which hath been spoken through the prophet Joel:" — Acts 2:16 (ASV)

This is that. This is the fulfillment of that, or this was predicted. This was the second part of Peter's argument to show that this was in accordance with the predictions in their own Scriptures.

By the prophet Joel (Joel 2:28–32). This is not quoted literally either from the Hebrew or from the Septuagint. The substance, however, is preserved.