A.T. Robertson Commentary John 1:33

"And I knew him not: but he that sent me to baptize in water, he said unto me, Upon whomsoever thou shalt see the Spirit descending, and abiding upon him, the same is he that baptizeth in the Holy Spirit." — John 1:33 (ASV)

He said (εκεινος ειπεν). Explicit and emphatic pronoun as in verse 8, referring to God as the one who sent John (verse 6).

With the Holy Spirit (εν πνευματ αγιω). "In the Holy Spirit." Here again one needs the background of the Synoptics for the contrast between John's baptism in water (John 1:26) and that of the Messiah in the Holy Spirit (Matthew 3:11; Luke 3:16).