Tenant Smoking Pot

[Larry K.]

So I get the following email from a tenant yesterday. This is a two family house. Also, smoking is prohibited per the lease

"Sorry to bother you but i meant to send this earlier today someone upstairs is smoking "pot" and i do not want my kids smelling it. Can you talk to the tenant so they can smoke somewhere else
Thank you"

How would you handle this? Have you ever had to handle this situation?