Orient, Jane M. Sapira's Art & Science of Bedside Diagnosis 5th Ed. Chapter 1: The Art of Clinical Examination. P5.
"My original reflex hammer from medical school was simply a round rubber ball about the diameter of a nickel, mounted on a handle. This illustrates the point that any hammer is satisfactory if the examiner knows where to strike the tendon. I have even used the diaphragm of a Sprague stethoscope, and a former resident, a Vanderbuilt graduate, used her knuckle. Some physicians use the tips of their fingers, a tool that is always available."
I don't think a citation is really necessary in this case. You're tapping a tendon. That's all. You don't need special equipment. I wouldn't make the same argument for a stethoscope.
But if you want, Sapira is a really fun textbook. It's even a fun read for a layperson.