By the authority vested in me as President by the Constitution and the laws of the United States of America, including sections 121 and 12304 of title 10, United States Code, I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation Atlantic…
The Washington Post is also reporting on it, but it is behind a paywall.
https://www.washingtonpost.com/national-security/2023/07/13/biden-pentagon-reservists-russia-ukraine/