I don't think that Fitzgerald ever uses the exact words that Daisy left Gatsby for money. It is understood that Daisy, whose name literally means a "day's eye," recognized the challenges with Gatsby and fled back to the comfortable and insulated world of Tom's wealth and prestige. When Gatsby waits for Daisy to return to him, thinking that she will respond to his act of moral courage, Fitzgerald describes him as waiting "in the moonlight—watching over nothing." This nothingness is enhanced when it becomes apparent that she is planning to take a long vacation with Tom, leaving Gatsby with the gravity of a situation that she caused. When Nick says that people like Daisy and Tom are "careless" people who "smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made," it becomes clear that Daisy chose Tom for money. She chose Tom because of her own moral cowardice and, in Tom, there is nothing real in terms of emotional commitment.
There is never a point in which Fitzgerald explicitly says that Daisy stayed with Tom for the money or that she left Gatsby because of money. If Fitzgerald says this, Daisy becomes a "gold digger" and rather one- dimensional. In constructing the description of why Daisy leaves Gatsby waiting as one in which she flees back into her world of wealth and privilege, it enhances Daisy's characterization, making her more human in her lack of moral courage. Her cowardice is what prevents her from reciprocating Gatsby's love. Daisy's love can be nothing more than a fleeting "day's eye." Her sense of love is one that is rooted in mirth, something lighter than "a fairy's wing," and is one that cannot compare to the weight with which Gatsby loves her. She left him for money and for privilege, but she left him out of moral cowardice and an inability to reciprocate a love so pure. In her abandonment on these grounds as opposed to merely money, Daisy becomes a figure of contempt, but only because she is so very human in such a regard.