Māṇḍūkī Śikṣā 22

22

na cāṅgulībhiraṅguṣṭhamupeyāddoṣavittataḥ
ūrdhvamāpustamākuñcamaṅguṣṭhaṃ sthāpayed budhaḥ

And one should not touch the thumb with the fingers, knowing the fault thereof; / the wise man should hold the thumb raised, curved inward, and bent.