You say she's autistic, and that makes a huge difference in where it's coming from. No doubt she can still be sassy, but she may not be able to learn to control herself like another child might after enough timeouts. My 11 yr old still occasionally gives me grief when I ask her to do something she doesn't want to do. But instead of repeating what I've said (though she has done that in the past), what she does is repeat lines picked up from books or movies. She has one for each situation, and I admit it drives me bonkers.
For her, I finally realized, it's some kind of deep-seated need to have the last word. Don't know if that's her way of completing the conversation or what, but since I've let it go when she says these things and don't challenge her, everything's been much better. At first, sometimes, she would get kind of mad when I wouldn't respond and would repeat the phrase again, because she has a script that we're supposed to stick to, and I wasn't sticking to it. Still does that sometimes, though not nearly as often. But mostly, it ends there as long as I let her have the last word. I mean, I don't care as long as she actually brushes her teeth or whatever! And the behavior has diminished with time. Now she does many things she used to complain about without a word back.
Also, with autistic kids, if they're doing a behavior you don't like, the best thing is to totally ignore it (as long as it's not causing any harm, of course!). They pick up real quick what their parents/caregivers don't like, and give it right back when you say something to them that they don't want to hear. The only way to break that is to not let them know it drives you batty. Very difficult, I know, and I wasn't always able to stick to it, but worth a try -- when we got good at ignoring her outbursts, they diminished. This is a hugely important step in modifying her behavior.
I agree with the person who said you need to speak to her with care -- don't know if this might be the situation with your daughter, but my daughter is extremely sensitive to tone of a person's voice, and often this will be what sets her off, regardless of the actual words said. It's hard for her to figure out nobody's mad because she can't grasp cues like other kids can.
We've had to make sure to use a cheery tone with her, and we usually end "requests" with "sweetie" or something like it. And then we give lots of thanks when she does something the way it should be done -- these encourage her tremendously and help her realize (I think) that we're not ordering her to do these things because she's being punished (which I think is how she feels when I tell her it's time to clean her room, or brush her teeth, or get up for school, etc).
You'd be surprised how far just a slight tone of voice change can go. We were never mean or yelling before, but it's hard to sound happy when you're annoyed with your kids for not listening. It's a big challenge, but worth it for the results! We have a lot less drama now. And everybody in the house talks to everyone else a lot nicer, which can't hurt. :)
