The Net Bible says this:
8:37 He said to him, If you believe with your whole heart, you may. He replied, I believe that Jesus Christ is the Son of God. Verse 37 is lacking in {Ì45,74 Í A B C 33 614 vg syp,h co}. It is clearly not a part of the original text of Acts. The variant is significant in showing how some in the early church viewed a confession of faith. The present translation follows NA27 in omitting the verse number, a procedure also followed by a number of other modern translations.
From Albert Barnes:
This verse is missing in a very large number of manuscripts (Mill), and has been rejected by many of the ablest critics. It is also omitted in the Syriac and Ethiopic versions. It is not easy to conceive why it has been omitted in almost all the Greek mss. unless it is spurious. If it was not in the original copy of the Acts , it was probably inserted by some early transcriber, and was deemed so important to the connection, to show that the eunuch was not admitted hastily to baptism, that it was afterward retained. It contains, however, an important truth, elsewhere abundantly taught in the Scriptures, that faith is necessary to a proper profession of religion.
And Adam Clarke:
This whole verse is omitted by ABCG, several others of the first authority, Erpens edit. of the Arabic, the Syriac, the Coptic, Sahidic, Ethiopic, and some of the Slavonic: almost all the critics declare against it as spurious. Griesbach has left it out of the text; and Professor White in his Crisews says, Hic versus certissime delendus, this verse, most assuredly, should be blotted out. It is found in E, several others of minor importance, and in the Vulgate and Arabic. In those MSS. where it is extant it exists in a variety of forms, though the sense is the same.