Daisy's decision to choose Tom's wealth instead of Gatsby's love is alluded to at various points in the story. One direct quote about the matter comes from the mouth of Gatsby himself.
In chapter seven of The Great Gatsby, tension has been building between Tom and Gatsby. The tension had already begun earlier in the day when Tom noticed the flirtatious interaction of Daisy and Gatsby before the group went to town. Tom feels further nettled when Gatsby comes to Daisy's defense, telling Tom to leave her alone after Tom has shown annoyance about Daisy's complaints about the heat. Tom begins to ask suspicious questions about Gatsby's past and credentials, which causes Daisy to stick up for Gatsby, much to Tom's further outrage. Tom indicates that he knows about Daisy and Gatsby's romantic behavior with one another, to which Gatsby reacts as a man whose honor has been threatened. Both men ignore Daisy's attempts to de-escalate the situation. Provoked by one another's challenging words, Gatsby and Tom have the following exchange:
‘Your wife doesn’t love you,’ said Gatsby. ‘She’s never loved you. She loves me.’
‘You must be crazy!’ exclaimed Tom automatically.
Gatsby sprang to his feet, vivid with excitement.
‘She never loved you, do you hear?’ he cried. ‘She only married you because I was poor and she was tired of waiting for me. It was a terrible mistake, but in her heart she never loved any one except me!’