"SMS" has proven to confuse some younger users in usertesting. The envelope icon is reserved for email. Is there an established or emerging standard that says "send this by text message"?
I think the use of a speech bubble is fairly common, however android seems to have stuck with an envelope (I'm an iOS man so forgive me if this is wrong)
A quick search shows mostly speech bubble related icons, some using text to further clarify.
If I were designing from scratch I'd consider a speech bubble coming from a mobile phone screen - this shows it's 'chatting' (text message) and that it's something done via mobile.
Quick mockup of that: