Answer: Always put the author's last name.
Explanation;
When citing a source in a paper the general consensus according to most referencing formats is to include the author's last name in a parenthetical in-text citation. The main reason for this is to ensure that the citation is easily located in the works cited page as well as to reduce interruption in the paper as much as possible.
In cases where the author is not known, the title of the source will usually suffice.