The phrase "unnamed sources" refers to information or statements provided by people who are not identified by name. It means that the source of the information is kept anonymous or undisclosed. Full definition
Our story was based on widespread media reporting, citing unnamed sources. (hacked.com)
At the time, the rumor was based on information from unnamed sources familiar with the matter. (automobilemag.com)
«Essentially, the majority of MPs will formally set up their own grouping under an alternative Labour banner,» one unnamed source told the newspaper. (politicshome.com)