The answer is: Civil Rights Act of 1964
Section 504 of the 1973 Rehabilitation Act was created to prohibit discrimination toward people who have disabilities in terms of receiving federal assistance or treatment in the work place.
Section 504 of the 1973 rehabilitation act was inspired by Civil Rights Act of 1964. But, Civil right Act of 1964 only protect discrimination based on race, religion, skin color, sex or national origin. Civil right act of 1964 did not mentioned disabled people. So, Section 504 of the 1973 rehabilitation act borrowed the language directly and applied it to the disabled.