A medical source statement is an important piece of evidence in a Social Security disability claim. The opinion of your treating doctor regarding your functional limitations should be given controlling weight by the SSA, if the opinion is not inconsistent with the other substantial evidence in the case record. 20 C.F.R. 404.1527(d)(2).
Social Security has a form, entitled Medical Source Statement of Ability to do Work-Related Activities (Physical), for a treating doctor to give an opinion regarding a patient's functional limitations for a Social Security claim. The form is not available on Social Security's website, and it is hard to find anywhere online.
Here is the version of HA-1151 that I use in my Social Security disability practice. Download form HA-1151.
Warning: be careful with this form! If your doctor gives Social Security an opinion regarding your limitations that still allows for employment at the sedentary exertional level, your claim could be worse off with that opinion than without it. It depends upon your particular situation. Do not have your doctor complete the form and blindly send it to Social Security. Get help from a Social Security disability attorney first.