In the fourth month Dhruva Mahārāja became a complete master of the breathing exercise, and thus he inhaled air only every twelfth day. In this way he became completely fixed in his position and worshiped the Supreme Personality of Godhead.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
चतुर्थमपि वै मासं द्वादशे द्वादशेऽहनि । वायुभक्षो जितश्वासो ध्यायन् देवमधारयत्
caturthamapi vai māsaṃ dvādaśe dvādaśe'hani | vāyubhakṣo jitaśvāso dhyāyan devamadhārayat
SB 4.8.75