Srimad-Bhagavatam: Canto 4 - Chapter 1 - Verse 75
Sanskrit:
चतुर्थमपि वै मासं द्वादशे द्वादशेऽहनि ।वायुभक्षो जितश्वासो ध्यायन्देवमधारयत् ॥ ७५ ॥
ITRANS:
caturtham api vai māsaṁdvādaśe dvādaśe ’hanivāyu-bhakṣo jita-śvāsodhyāyan devam adhārayat
Translation:
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.