Acts 8:37 King James Version And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
Many modern versions of the Bible do not include Acts 8:37, and/or have a note that some Bibles include it, or that it was not in the oldest manuscripts. The New American Standard has this note: "Early mss do not contain this verse."
For example, the New International Version:
Acts 8: 36 As they traveled along the road, they came to some water and the eunuch said, “Look, here is water. What can stand in the way of my being baptized?”  [c] 38 And he gave orders to stop the chariot. Then both Philip and the eunuch went down into the water and Philip baptized him.
It is true that these words are omitted in old manuscripts such as Sinaiticus (4th century), Vaticanus (4th century) and Alexandrinus (5th century). The Codex Laudianus, also known as Ea or 08 contains an almost complete text of the Book of Acts. It is the oldest or earliest known manuscript which contains the words of Acts 8:37. This manuscript is dated to the 6th century.
Nevertheless, there are two sources for the reading which are earlier than the manuscripts that omit it. This indicates it was in some early manuscripts not now extant, and part of the original writing of Luke.
In Against Heresies (3.12.8), Irenaeus of lyons (circa AD 180) makes a reference to this as scripture:
8. But again: Whom did Philip preach to the eunuch of the queen of the Ethiopians, returning from Jerusalem, and reading Esaias the prophet, when he and this man were alone together? Was it not He of whom the prophet spoke: "He was led as a sheep to the slaughter, and as a lamb dumb before the shearer, so He opened not the month? ""But who shall declare His nativity? for His life shall be taken away from the earth." [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."About 70 years later Cyprian (circa AD 250) mentions the first part of what we know as verse 37 (found in The Treatises of Cyprian, Treatise 12, Book 3.43):
43. That he who believes can immediately obtain (i.e., pardon and peace). In the Acts of the Apostles: Lo, here is water; what is there which hinders me from being baptized? Then said Philip, If you believe with all your heart, you may.In addition to these, in the Life and Passion of St. Cyprian Pontius the Deacon alludes to Philip believing with "his whole heart" -- which comes from this text.
3. The apostle's epistle says that novices should be passed over, lest by the stupor of heathenism that yet clings to their unconfirmed minds, their untaught inexperience should in any respect sin against God. He first, and I think he alone, furnished an illustration that greater progress is made by faith than by time. 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.These three references combined show that these words were known by early Christian writers, indicating manuscripts such as Sinaiticus and Vaticanus omit something which was originally there rather than Laudianus adding something that was not.