In Graduation Day, Part Two (S3E22) there's a dream sequence with Faith:

BUFFY: Who's going to look after him? FAITH: It's a she. And aren't these things supposed to take care of themselves? BUFFY: A higher power guiding us? FAITH: I'm pretty sure that's not what I meant. BUFFY There's something I'm supposed to be doing. FAITH Oh yeah. Miles to go. Little Ms. Muffet counting down from 7-3-0. BUFFY: Great. Riddles.

"Little Ms. Muffet" is a reference to Dawn - the 730 is the number of days until The Gift (this is confirmed by David Fury & Joss in the season five featurette, Spotlight on Dawn). This gets referenced later (after Dawn arrives) when a crazy man in Real Me (S5E02) tells Dawn she is "curds and whey".

In the opening of This Year's Girl (S4E15), we again have a dream sequence with Buffy and Faith:

BUFFY: Smell good, don't they? (They are making a bed, in the bedroom where we first see Dawn) FAITH: What? BUFFY: Clean sheets. Like summer. FAITH: I wouldn't know. BUFFY: Right. I forgot. FAITH: I noticed. BUFFY: I wish I could stay, but- (Buffy notices the clock, which shows 7:30) FAITH: You have to go. BUFFY: It's just, with… FAITH: Little sis coming. I know. So much to do before she gets here.

In Restless (S4E22), it's Buffy and Tara, in the bedroom again:

BUFFY: Faith and I just made that bed. TARA: For who? BUFFY: I thought you were here to tell me. The guys aren't here, are they? We were gonna hang out and, watch movies t- TARA: You lost them. BUFFY: No. No. I think they need me to find them. (We see the clock again showing 7:30) BUFFY: It's so late. TARA: Oh ... that clock's completely wrong. Here. BUFFY: I'm never gonna use those. TARA: You think you know ... what's to come ... what you are. You haven't even begun. BUFFY: I think I need to go find the others. TARA: Be back before Dawn.

This is the episode immediately before Dawn is introduced.