A medical source statement is an opinion from your doctor regarding the nature and severity of your limitations due to your medically-determinable impairments. A medical source statement is often simply a form that is completed and signed by your doctor setting forth the doctor’s opinion regarding your functional limitations.
The SSA must consider your treating doctor’s opinion. See 20 CFR 404.1527.
Here are the forms I use. The first form is used to describe physical limitations. The second form is used to describe mental limitations.