What is the evidence that a woman shortens her hair by the length of a fingertip upon exiting the state of Ihram?

Shaving or shortening the hair is a ritual in Hajj and Umrah. However, a woman does not exit the state of ihram by shaving, as it is a form of mutilation for her. Rather, she shortens her hair, based on the saying of the Messenger of Allah, peace and blessings be upon him: "There is no shaving for women; for women there is only shortening." The amount to be shortened for a woman is estimated to be approximately the length of a fingertip, not precisely defined, and any amount she shortens will suffice her. Some scholars are of the opinion that taking three hairs or more is sufficient.