Bhagavatapurana 11.14.35

11.14.35

evaṃ praṇava-saṃyuktaṃ prāṇam eva samabhyaset /
daśa-kṛtvas tri-ṣavaṇaṃ māsād arvāg jitānilaḥ //

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.