As reported by Censor.NET, this is stated in the report "Russia's War in Ukraine: The Medals and Treacherous Numbers" published by Bellingcat open source and social media investigation group.
The investigation deals with the imagery of medals awarded to Russian servicemen. Most of the higher Russian medals have a consecutive numbering, explicitly stating the number of medals awarded so far.
According to Bellingcat, the number of awarded medals, compared to the years before 2014, suddenly and strongly increased in 2014 and 2015. The large number of awarded medals "For Distinction in Combat", 4300 awards between Nov. 7, 2014 and Feb. 18, 2016, strongly suggests larger combat operations with active Russian military involvement in this period. In sum, the data suggests that more than ten thousand medals of four types were awarded in the considered period.
"Therefore, it can be directly concluded that thousands of Russian servicemen participated in 2014 and 2015 in combat operations and were awarded with medals for their actions in these operations," the report notes.
As it is a rather unrealistic assumption that every Russian serviceman participating or involved in the fighting in eastern Ukraine has been awarded with a medal, it is possible to conclude "most likely tens of thousands Russian servicemen participated in or contributed to the fighting in eastern Ukraine."
Earlier, Bellingcat released fresh satellite imagery proving the ingress of Russian military hardware in Ukraine.