चतुर्युगाख्यान (Caturyuga-Ākhyāna) — Yuga-wise Origins and Measurements of Beings
सामान्येषु तु धर्मेषु तथा वैशेषिकेषु च / ब्रह्मक्षत्रविशो यस्माद्युक्तास्तस्मा द्द्विजातयः
sāmānyeṣu tu dharmeṣu tathā vaiśeṣikeṣu ca / brahmakṣatraviśo yasmādyuktāstasmā ddvijātayaḥ
In the common duties and in the specific duties as well, since Brahmins, Kshatriyas, and Vaishyas are joined to them, therefore they are called the twice-born (dvijāti).