Aśauca-vidhi (Rules of Impurity) within Śrāddha-kalpa — Chapter on Testing/Selecting Brahmanas and Honoring the Atithi
वृत्तिरेषा तु वैश्यस्य ब्राह्मणस्य तु पातकम् / आहरेद्भृतितो वेदान् वेदेभ्यश्चोपजीवति
vṛttireṣā tu vaiśyasya brāhmaṇasya tu pātakam / āharedbhṛtito vedān vedebhyaścopajīvati
This livelihood may befit a vaiśya, but for a brāhmaṇa it is a sin: he who procures/teaches the Veda for wages and lives off the Veda is at fault.