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)
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.