Mimamsasutra 3.2.13

He shows another inquiry:
Is the conduct of the virtuous an invalid source or not? It is invalid for lack of a root [in the Veda];
It is a means of valid cognition because, like the Aṣṭakā rites and the like, it has a root [in the Veda].

If it is argued that the conduct of the virtuous, such as the Holāka festival and the like, lacks validity because of the absence of a root Veda—no; for, because it has been accepted by orthodox learned people, it has its root in the Veda like the Aṣṭakā rites and the rest. For this very reason, even though not taught in a specific form by Manu and others for fear of text elongation, the conduct of the virtuous has been taught in a general form, through the citation of means of valid cognition regarding dharma in the manner of: ‘Direct Vedic statement, tradition, and the conduct of the virtuous.’ Therefore, the conduct of the learned is a means of valid cognition.

(In the fifth topical section on the validity of words established in authoritative treatises [in the section on the Ārya and Mleccha], aphorisms 8–9)