The electronic configuration of a neutral iron atom is [Ar] 3d6 4s2, so losing two electrons to form the Fe(II) ion leaves the valence configuration as [Ar] 3d6. This is a matter of basic definition in inorganic chemistry and does not require citation.
The claim states the fundamental electron configuration of the Fe(II) ion. This is a basic chemical fact taught universally in general and inorganic chemistry, making it common knowledge by definition and everyday scientific observation.