Source is
this interview with him and Feige from September, when the AoU DVD came out. Here's the full quote:
I did see the interview in september - was the same, as he talked about the **** gauntlet
As he said - they do not know NOW, which part ends. So there is no need to speculate.
Everything is still possible...
Evans wants to return, and "Cap 3" will crush it...
RDJ said yesterday the MCU is the best what could happen to him.
He will be from now till 2019 in one movie each year (2017 in Spiderman).
And i am sure - as i heard him yesterday talking - that, if Marvel want to, he will be around at least till phase 5 starts...
And with Thor - i think it will also be easy to keep Chris...
I do not see any changes here...
If, they will kill some guys like Hawkeye, Winter Soldier, Warmachine...
I am still sure, that we will see a fouth movie from "the big three"...
There is no need not to do that - especially, as we may have the next big thing (enemy) waiting - for sure that will be not without Iron Man, Captain America and Thor!!
They will exit some minor characters (Hawkeye -> family, Warmachine -> retired, Winter Soldier -> "finding himself"), then we have some room for Doctor Strange, Black Panther, Captain Marvel.