The evidence for the fact that after shaving the head or cutting the hair, a person's hair does not become impure.

Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates that when the Messenger of Allah (peace and blessings be upon him) had stoned the Jamrah ‘Aqabah and slaughtered his camel, he gave the barber the hair from the right side of his head, and he shaved it. Then the Prophet (peace and blessings be upon him) gave that hair to Sayyiduna Abu Talhah (may Allah be pleased with him). Then he gave the barber the hair from the left side, and he shaved it as well. Then the Prophet (peace and blessings be upon him) gave this hair also to Sayyiduna Abu Talhah (may Allah be pleased with him) and instructed him to distribute the hair among the people.