Bhagavatapurana 11.14.36

11.14.36

hṛt-puṇḍarīkam antaḥ-stham ūrdhva-nālam adho-mukham /
dhyātvordhva-mukham unnidram aṣṭa-patraṃ sa-karṇikam //

35. & 36. Thus he should exercise the breath along with Pranava ten times at each of the three parts of a day and within a month he will attain control over his breath: he should think the heart which is within the body like a lotus bud with its stalk upwards and itself turned downwards, is reversed i. e., the bud is turned upwards and open, having eight petals and a pericarp.