Income requirements for tenants in MA

[Manikantan Nambi]

Hi, I am closing on my first house (multi-family) in a week and I will be renting out one of the units. I have seen on BP and other websites that an income of 3-4x is a good rule of thumb while screening for tenants. My question is how do I enforce it? Can I tell potential tenants that I will only rent out to tenants that earn > 3x of the rent? Would that be considered discrimination? 

Thanks.