They weren't together at this time, but i always liked the moment in Older And Far Away, when Tara stands up to Anya, who wants Willow to use magic.
TARA: She said no, and that's it. You're not gonna make her do something that she doesn't want to. And if you try... (folds arms across her chest) You're gonna have to go through me first. Understood?
And then another great moment follows:
WILLOW: Just so you know, I-I was never ... gonna use it. I mean, not really, I ... I just kept it like a safety net. 'Cause ... there was always this ... thing in the back of my head. This, you know, voice, saying, like, 'what if things get bad, I mean really bad? And what if you can't handle it?' A-and it made me panic, so ... that's why I kept a couple of things. I ... kept them so I-I didn't have to think about it, so I could focus on ... on getting better.
TARA: I get it. I-I really do. But it's time to work without the net, Will.You know, I don't know if you noticed, but it actually did get bad in there. Really bad, and ... and you still said no.
Yup, Tara Rules.