The entire episode The Zeppo
His speech to Buffy in Into The Woods:
Xander: You gonna let him go?
Buffy: It's not my decision to make.
Xander: Of course it is.
Buffy: Well, it's not fair!
Xander: Who cares if it's fair? In about twenty minutes, Riley's gonna disappear, maybe forever, unless you do something to stop him.
Buffy: What am I supposed to do? Beg him to stay?
Xander: Why wouldn't you? To keep Riley here you—
Buffy: I don't even know who he is anymore! I mean, I thought he was... dependable.
Xander: Dependable? What is he, State Farm?
Buffy: You know what I mean.
Xander: Yeah. I think you mean convenient. I think you took it for granted that he was gonna show up when you wanted him to, and take off when you didn't.
Buffy: Look who's talking. Look who has Anya following him around like a lovesick puppy.
Xander: Oh boy, is this not about me.
Buffy: Is she more than a convenience? 'Cause that would kinda be a surprise!
Xander: (angry) If you don't wanna hear what I have to say, I'll shut up right now.
Buffy: Good, 'cause I don't.
Xander: I lied. See, what I think, you got burned with Angel, then Riley shows up.
Buffy: I know the story, Xander.
Xander: But you missed the point. You shut down, Buffy. And you've been treating Riley like the rebound guy. When he's the one that comes along once in a lifetime. He's never held back with you. He's risked everything. And you're about to let him fly because you don't like ultimatums? If he's not the guy, if what he needs from you just isn't there, let him go. Break his heart, and make it a clean break. But if you really think you can love this guy... I'm talking scary, messy, no-emotions-barred need... if you're ready for that... then think about what you're about to lose.
Buffy: Xander...
Xander: Run.
Plus when he's being nice to Anya in a few eps 😛