He said words that would be remembered forever One small step for man, one giant leap for mankind
Here is a possible answer with the reasons below. He said words that would be remembered forever: “One small step for man, one giant leap for mankind.”
1. The first half of the sentence is a complete sentence speech tag.
That is, it is a complete sentence that could stand alone, but it could also be considered the speech tag for the quoted words. When a speech tag could stand alone, it is one of the few times that a colon is appropriate following a speech tag. (Note: You could also just consider it a stand-alone sentence and follow it with a period and have no “speech tag.”)
(It is not proper to follow a non-sentence/short speech tag with a colon. A comma should be used in that case.)
2. The beginning quotation mark comes just before the quote begins–before One.
3. The ending quotation mark comes after mankind–
However, the period for the entire sentence goes INSIDE the closing quotation mark as all periods (and commas) go inside closing quotation marks in the US (but not in UK).
4. Internal Comma
As for the internal comma (one small step for man, one giant leap for mankind), that is how the quotation is written almost everywhere it is written; thus, whenever you write a quote, you should write it word-for-word as it was when you researched it/found it. Some would say that should be a semicolon. It is probably more accurate to consider it a comma before an “understood” and. Either way, it is a quote and will remain with its original punctuation.