In contemporary mathematical logic, we generally ignore some philosophical distinctions. My guess is that you do not mean "proposition" here as the label that sits beside some theorems in a book, and instead you mean it in a philosophical sense.
In this sense, a statement expresses a proposition, but the same proposition could be expressed by multiple statements. A theorem is then a statement that has been proved. Or is it a proposition that has been proved? You can see that there are many complications here.
The existence, or lack thereof, of abstract "propositions", independent of the sentences that express them, is an important topic in philosophy. So you may have encountered a book that takes a more philosophical approach and mentions the distinction. But, especially at first, you can likely just identify propositions with statements, and everything will be fine.