The passage you are looking for is Acts 8:27-31 and it reads as follows:
And he rose and went. And behold, an Ethiopian, a eunuch, a minister of the Candace, queen of the Ethiopians, in charge of all her treasure, had come to Jerusalem to worship and was returning; seated in his chariot, he was reading the prophet Isaiah. And the Spirit said to Philip, “Go up and join this chariot.” So Philip ran to him, and heard him reading Isaiah the prophet, and asked, “Do you understand what you are reading?”
And he said, “How can I, unless some one guides me?” And he invited Philip to come up and sit with him.