It's спрашивать, not спросить. But anyway, I don't specifically know the answer to your question. From what I've seen, the с- prefix added to a verb usually adds the "from (off)" meaning of с (rather than the "with"meaning) to the verb. I can't confirm whether that's definitely what's happening here. просить and спросить clearly have very similar meanings and it's hard to pin down the subtle differences between the two.