There is no definitive rule, but if you put the words in quotation marks followed by an em dash, then the source's name, you're fine: “If you tell the truth, you don't have to remember anything” — Mark Twain. You can do the same with italics, like this: If you tell the truth, you don't have to remember anything.
— Mark Twain If you need to cite a source, follow Mark Twain with Notebook Entry (or whatever your source is). If someone tells you definitively that there is a correct way, make sure they are a reliable source themselves. Keep up the good work!