The answer to this question depends on which copy of the book you're referring to. In the ebook version of To Kill a Mockingbird that I have it's on page 42 that Scout mentions Dill's asking to marry her earlier in the summer.

Dill's way too young to have much idea about love and marriage; at best, his feelings for Scout are no more than an expression of puppy love. Scout's also too young to comprehend the true meaning of marriage, but she's old enough to know that Dill's not being serious. Despite asking Scout to marry him, and telling her she's the only girl he'd ever love, he promptly neglects her, and forgets about the whole thing. Dill's getting to be a real pain in the neck, so much so that Scout even beats him up a couple of times. But it's all to no avail, as Dill continues to hang around, drawing closer to Jem as the two boys spend all their time together in the treehouse, plotting and planning.