Māṇḍūkī Śikṣā 23

23

nādhaḥ śirastāddāme vā nāṅgulyaḥ pratarāḥ smṛtāḥ
uttānaṃ sonnataṃ kiñcitsuvyaktāṅgulirañjitam

Not below the head, nor to the left, nor should the fingers be stretched too far apart; / [the hand should be] upturned, slightly elevated, and graced with distinctly separated fingers.