Maxwell's first equation:
Source: https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Electricity_and_Magnetism_(Tatum)/15%3A_Maxwell's_Equations/15.02%3A_Maxwell's_First_Equation
The surface integral of the electrostatic field D, over a closed surface is equal to the charge enclosed by that surface.