Some useful references are given below, readily available online (with access to JSTOR). Weisfeiler's short paper was published just before his disappearance while hiking in Chile in January 1985, but some details have circulated in manuscript form. (Igor Pak has just now posted an overlapping account.) The original source is the paper by C. Jordan, J. Reine Angew. Math. 84 (1878), 89–215].
MR0200350 (34 #246) 20.65 Brauer, Richard; Feit,Walter, An analogue of Jordan’s theorem in characteristic p. Ann. of Math. (2) 84 1966 119–131.
MR758425 (85j:20041) 20G20 (20D05) Weisfeiler, Boris (1-PAS), Post-classification version of Jordan’s theorem on finite linear groups. Proc. Nat. Acad. Sci.U.S.A. 81 (1984), no. 16, Phys. Sci., 5278–5279.