MLA in-text citation

This is regarding MLA in text-citation. I remember learning years ago that after you have cited a source either parenthetically at the end of the quote or within the text as you introduce the quote, you do not have to cite it again every time you quote the same source -- you just cite the page number within a parenthesis. For example, the first quote might end (Myles 21), the second quote (33) and the third quote (90). Only when you quote a new source (Craig 20) and then quote again the previous source do you need to cite again this source (Myles 100). This is what I remember learning. However, I've been looking for this specific MLA rule on the internet and cannot find it. Has the rule changed or did I have it wrong all along? If the rule is in fact that you do not have to mention the source after the first citation -- just the page number --, please include the website or book info where I can locate it. Thank you very much!