[Your Name, & Address here]
[Recipient Name & address here]
I have received a complaint about you that you [state complaint] on [date].
I was told that – [describe the incident in brief] –
I want to say that this is a serious issue that needs to be addressed. I cannot allow you to – [state what the tenant should not do] – while you are living in my property.
So, I advice you to [state what the tenant should do], and I hope I won’t receive any such complaints about you in the future.
In case there is a misunderstanding, I would like to hear your part of the story.
Thanks for your understanding.