You can find a proof in Corollary 5.3 in Krylov.

http://www.aimspress.com/data/article/preview/pdf/62a32789ba35de15514969d5.pdf

You can find a proof in Corollary 5.3 in Krylov's On parabolic Adams's, the Chiarenza-Frasca theorems, and some other results related to parabolic Morrey spaces (Link)

Here is also a link to the PDF of the article.