'assertion by' is correct and is used in written English
You can use it when something is being attributed to a particular person. For example: "This assertion by the Prime Minister has caused considerable controversy."
Sather quickly followed that assertion by saying: "Who knows?
I was astonished at the assertion by a Feb.
Taguba backed up his assertion by citing evidence from sworn statements to Army C.I.D. investigators.
One assertion by President Obama about energy achievements in Tuesday night's campaign debate drew little notice.
And so it went on... assertion by Jay, denial by Murdoch.
He rejected an assertion by Mr. Sheikh that Mr. Pearl, 38, was dead.
Rather, the central assertion by the plaintiffs is that the Obama Administration violated the law itself.
Enron said it was reviewing the "assertion" by Dynegy that it could claim the pipeline.
He does not disagree with that assertion by another scholar, David M. Goldenberg.
Mr. DiFrancesco said he was shocked at the assertion by Mr. Markowitz.
The government of President Laurent Kabila denied the assertion by the rebel leader, Jean-Pierre Bemba.
