May be in this book you can find what you are looking for:
Theory and Applications of Fractional Differential Equations, by Kilbas, A. A.; Srivastava, H. M.; and Trujillo, J. J. Amsterdam, Netherlands, Elsevier, Febrary 2006. ISBN 0-444-51832-0 (http://www.elsevier.com/wps/find/bookdescription.cws_home/707212/description#description)
You can check Theorem 3.15 of Chapter 3 of the book: The Analysis of Fractional Differential Equations, authored by K. Diethelm, and edited by Springer-Verlag Berlin Heidelberg in 2010.