The transformation laws for electric and magnetic fields prescribed by special relativity (in MKS) for a particle traveling at a speed v are
Four-Vector, Eric Weisstein's World of Math Lorentz Transformation Eric Weisstein's World of Math
References
Griffiths, D. J. Introduction to Electrodynamics, 3rd ed Englewood Cliffs, NJ: Prentice-Hall, pp. 437-443 and 445-448, 1981.