Freebbble!

iPhone 6 Flat + Sketch / Freebie AI is a free Mockup design resource made by Dribbble user Georgian-Sorin Maxim. As far as I can tell, its license is Unknown.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.