The regulation would be 21 CFR Part 117.
You would need to determine whether you fall under an exemption for the Preventive Controls Rule. If you are not exempt, than it is dependent on your food safety plan.
117.165"...Environmental monitoring, for an environmental pathogen or for an appropriate indicator organism, if contamination of a ready-to-eat food with an environmental pathogen is a hazard requiring a preventive control, by collecting and testing environmental samples"
So if you identified an environmental pathogen risk needing a preventive control then you are required to have environmental monitoring.