To quote directly:
"To be canceled, this process requires the presence of at least one reliable source that supports at least one statement made about the person in the biography. Do not remove the  until the biography has at least one such source."
However, point two of the same policy says:
"2. however, in borderline cases, such as where a source of arguable but questionable reliability has been added, the biography should be listed instead at Articles for Deletion."
In short, if there is a source in the article but its reliability is being questioned, unless there is clear consensus that the source is not reliable, an AfD is preferable to a BLPPROD.