That an astronaut crossing the event horizon of a black hole experiences extreme tidal forces is a standard consequence of general relativity well-known by definition and everyday instruction in physics, requiring no specific citation.
The claim states a foundational, textbook physical consequence of general relativity regarding black hole event horizons and tidal forces. Although specialized papers discuss tidal forces and geodesic deviation in various metrics, the core principle is a matter of basic physical definition and observation within gravitational theory, making it common knowledge for which specific evidentiary citations are unnecessary.