Actually there is in the new FAQ, they added this section (take effect 4/2/2016)
That's not really rules about how to roll the dice, just what to do if you roll too many or too few.
The OP wanted a rule he could point out to the TO apparently so he'd stop rolling one die at a time.
I agree rolling one at a time could be considered stalling, but that's a judgement call not a hard and fast rule.
But that is a rule that solves his issue of them rolling one die at a time. It's also the only rule we have about rolling dice.
Rolling one die at a time falls under the rolling too few rules, and one player could call the TO about it.
That being said, i don't think i ever would, because while it is the letter of the rules, it just leaves a bad taste in my mouth personally. Then again so does rolling one die at a time.the large issue for me are those people who launch their dice like missiles across the play mat ... that drives me bonkers. But that is another topic lol.