Most Hindus are familiar with the term "Sanātana Dharma" which is used in several scriptures. E.g., Manusmṛti 4.138 uses the term as follows:
satyaṃ brūyāt priyaṃ brūyānna brūyāt satyamapriyam |
priyaṃ ca nānṛtaṃ brūyādeṣa dharmaḥ sanātanaḥ || 138 ||
He shall say what is true; and he shall say what is agreeable; he shall not say what is true, but disagreeale; nor shall he say what is agreeable, but untrue; this is the eternal law. — (138)
But why is Sanātana Dharma also called Ārṣa Dharma (आर्ष धर्म) sometimes?
Is it a colloquial term or actually used as such in any of our scriptures?
If so, what are some scriptures that use the term ārṣa dharma?