Lemma on fibrations

I'm looking for a reference for the following lemma on composition of fibrations Let p:E->B and r:B->A be fibrations and D be a subcategory of A-> stable under pullback. If r has indexed products along morphisms in D and if p has indexed products along all cartesian maps above something in D then the composition rp has indexed products along D, too and the Beck-Chevalley condition holds. Martin Hofmann, Edinburgh ==============================================================================