Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 35
Sanskrit:
एवं प्रणवसंयुक्तं प्राणमेव समभ्यसेत् ।दशकृत्वस्त्रिषवणं मासादर्वाग् जितानिल: ॥ ३५ ॥
ITRANS:
evaṁ praṇava-saṁyuktaṁprāṇam eva samabhyasetdaśa-kṛtvas tri-ṣavaṇaṁmāsād arvāg jitānilaḥ
Translation:
Being fixed in the oṁkāra, one should carefully practice the prāṇāyāma system ten times at each sunrise, noon and sunset. Thus, after one month one will have conquered the life air.