The word "greed" doesn't appear in the play itself: though that it's certainly not irrelevant to "Macbeth" and its themes. More usually critics and scholars talk about the theme of ambition in the play, rather than greed.
But there are a few quotes that might be of use. Macbeth spends a lot of time thinking about his "black and deep desires" for power:
Or you might look at what he says after Malcolm is made the Prince of Cumberland:
And Angus sums up quite well the uneasiness of someone who has grabbed and grabbed - and then been left in a very insecure position:
His secret murders sticking on his hands;
Now minutely revolts upbraid his faith-breach;
Those he commands move only in command,
Nothing in love: now does he feel his title
Hang loose about him, like a giant's robe
Upon a dwarfish thief.
Macbeth has grabbed a title which is way too big for him: as if a dwarf has stolen a giant's clothes. That, I think, is about the best indicator of Macbeth's greed and ambition, in the play.
Although we can identify greed in Macbeth, the word doesn't actually appear in this play. Rather, ambition (which is after all a kind of greed) is much more prevalent. We know that Macbeth has "black and deep desires" from his first soliloquy and it is clear that Lady Macbeth, his "partner in greatness" has high ambitions for her husband too.
One of the key quotes has to be Lady Macbeth speaking to her husband and dangling future possibilities in front of him:
Glamis thou art, and Cawdor, and shalt be
What thou art promised.
In addition, Macbeth states: "I have no spur to prick the sides of my intent, but only vaulting ambition, which o'erleaps itself, and falls on the other." It is this greed or ambition that drives the play and results in such a high body count.