Testimony from 2 church Fathers showing Acts 8:37 was in the original text

“[Philip declared] that this was Jesus, and that the Scripture was fulfilled in Him; as did also the believing eunuch himself: and, immediately requesting to be baptized, he said, “I believe Jesus Christ to be the Son of God.” This man was also sent into the regions of Ethiopia, to preach what he had himself believed, that there was one God preached by the prophets, but that the Son of this [God] had already made [His] appearance in human nature (secundum hominem)” (Irenaeus)-IRENAEUS AGAINST HERESIES 140 – 200 BOOK III.

“For although in the Acts of the Apostles the eunuch is described as at once baptized by Philip, because “he believed with his whole heart,” this is not a fair parallel. For he was a Jew, and as he came from the temple of the Lord he was reading the prophet Isaiah,” (Cyprian)-Cyprian THE LIFE AND PASSION OF CYPRIAN, BISHOP AND MARTYR 200 – 258 paragraph 3