$\nabla \cdot \mathbf {B} =0$
Integral form: $\unicode{x222F}\,{\mathbf {B}}\cdot {\mathrm {d}}{\mathbf {A}}=0$ the net flux of the magnetic field out of a closed surface is zero
Differential form: $\nabla \cdot \mathbf {B} =0$ magnetic field has zero divergence.