There are many.
"Fair is foul and foul is fair" by the witches, then Macbeth "So fair and foul a day." They immediately link Macbeth with the witches and their prophecy.
"Out damned spot" by Lady Macbeth as she sinks into insanity as a result of her role in Duncan's murder.
"By the pricking of my thumbs something wicked this way comes" by the witch as Macbeth approaches, indicating what Macbeth has become and what his inevitable fate will be.
There are also their prophecies relating to what Macbeth takes as assurances but in fact are confirming his death. "Till Birnam wood remove to Dunsinane" and "No man born of woman" can harm him. In the former Macbeth's foes cut branches from Birnam wood as they approach Macbeth's castle, and of course Macduff whose family Macbeth has slaughtered was born by caeserean.
There are many quotes in Macbeth that can be used to answer different questions.
For example, Macbeth's soliloquy beginning 'Is this a dagger I see before me...' can be used to answer a question about Macbeth's conscience/guilt, as can the quote 'Macbeth dost murder sleep' about the terrible deed he has done.
It is also important to look at quotes that can be linked to one another, like when Macbeth paraphrases the witches about fair being foul and foul being fair. These can help to answer questions about important themes in the play.
Soliloquys, or at least key parts of them, are very good sources to focus on. There are many throughout Macbeth, spoken by many different characters. Using parts of these soliloquys as quotes can become a starter in order to explain, in your own words, what else happens that is linked. This technique also avoids overusing quotes.