One verse

Srimad Bhagavatam 2.1.17

2.1.17

अभ्यसेन्मनसा शुद्धं त्रिवृद्ब्रह्माक्षरं परम् । मनो यच्छेज्जितश्वासो ब्रह्मबीजमविस्मरन् ॥ १७ ॥

abhyasen manasa suddham trivrd-brahmaksaram param mano yacchej jita-svaso brahma-bijam avismaran

abhyaset
one should practice
manasa
by the mind
suddham
sacred
tri-vrt
composed of the three
brahma-aksaram
transcendental letters
param
the supreme
manah
mind
yacchet
get under control
jita-svasah
by regulating the breathing air
brahma
absolute
bijam
seed
avismaran
without being forgotten.

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.