What to do with this “rumor”

I heard a “rumor”, from my husbands mouth, that my mother called him telling him that she heard he was cheating on me with his mother’s friend. He is barely telling me this and when my mother called it happened about 4-5 months ago. He told me that to trust him and that those are just rumors. This blowed up because tomorrow it is his parents anniversary and his “lover” is hosting. I just don’t know what to do or who to believe. Please give me advice on what to do. 😫

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Update: I’m currently at the anniversary party and I don’t see any awkwardness or funny vibes between both of them. I did confront my mom in why she did not tell me about the rumor. She said that this might’ve happened when he was a teenager, but he has not done while he was dating me. I am going to act like I don’t know anything, but I’m also not going to let my guard down. Thank you guys so much for your advice, it really helped❤️