Albert Barnes Commentary Acts 1:5

"For John indeed baptized with water; but ye shall be baptized in the Holy Spirit not many days hence." — Acts 1:5 (ASV)

For John truly baptized, etc. These are the words of Jesus to his apostles; and he evidently has reference to what was said of John's baptism compared with his own in Matthew 3:11; John 1:33. In those verses John is represented as baptizing with water, but the Messiah who was to come as baptizing with the Holy Ghost and with fire. This promise respecting the Messiah was now about to be fulfilled in a remarkable manner. See Acts 2.

Not many days hence. This was probably spoken not long before his ascension, and of course not many days before the day of Pentecost.