“Free to use for whatever you want, except re-selling.”
Beautiful Messages iOS Icon is a free Icon design resource made by Dribbble user Christophe Tauziet. As far as I can tell, its license is Free for Commercial and Personal Purposes.
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.