It may not be. If it isn't, here is the scripture...
37 Then Philip said, “If you believe with all your heart, you may.”
And he answered and said, “I believe that Jesus Christ is the Son of God.” (NKJV)
Why is this scripture missing from several Bible translations?
I really don't know. Just being honest. Some scholars say it was never in the original text of the Bible and say it was added in. Some people even say it's a conspiracy.
Here is my take on it...
Added in or not, this scripture is the eunuch's proclamation of his faith. He believed with all his heart that Jesus is the Son of God.
Anyone who has had a moment like the eunuch did with Philip can pinpoint the day and place where they made their public declaration of their faith. It's that powerful.
This is not a scripture you should commit to memory. What you should commit to memory is when you realized Jesus is the Son of God and you believed it with all your heart.