Yājñavalkya Śikṣā 56

56

muṣṭyākṛtirmmakāre tu nakāre tu nakhagrahaḥ
anusvāre'ṅgulikṣepa ūṣmānte'ṅgulimokṣaṇam

On the sound m, the form of a fist; but on the sound n, grasping with nails; / on the pure nasal sound, throwing forward of fingers; at the end of a spirant, releasing of fingers.