What is the authenticity of the Hadith: "Two sins are unforgivable: associating partners with Allah and harming people"?

Allah does not forgive polytheism (shirk) except through repentance, unlike all other sins which He may forgive without repentance. As for harming people (injustices), Allah's right in them is forgiven through repentance, but the rights of the servants remain, and they will be avenged for them on the Day of Judgment, unless the oppressor strives in repentance and removes the effects of his injustice in this world.