After sitting in the above manner, make the mind remember the three transcendental letters [a-u-m], and by regulating the breathing process, control the mind so as not to forget the transcendental seed.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
अभ्यसेन्मनसा शुद्धं त्रिवृद्ब्रह्माक्षरं परम् । मनो यच्छेज्जितश्वासो ब्रह्मबीजमविस्मरन्
abhyasenmanasā śuddhaṃ trivṛdbrahmākṣaraṃ param | mano yacchejjitaśvāso brahmabījamavismaran
SB 2.1.17