How to get my ex to tell me how he caught me out?

So my boyfriend broke up with me about week ago because he found out I cheated.

When he confronted me I admitted to it, took full responsibility and apologised tearfully and promised it would never happen again, but he still broke up with me despite my obvious sincerity, fine I accept that, his choice

BUT he won't tell me how he found out, someone must have told him and he won't tell me who, do I not have a right to know who ratted me out?

How to get my ex to tell me how he caught me out?
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