SB 4.8.75

caturtham api vai māsaṁ
 dvādaśe dvādaśe ’hani
vāyu-bhakṣo jita-śvāso
 dhyāyan devam adhārayat
Synonyms: 
caturtham — fourth; api — also; vai — in that way; māsam — the month; dvādaśe dvādaśe — on the twelfth; ahani — day; vāyu — air; bhakṣaḥ — eating; jita-śvāsaḥ — controlling the breathing process; dhyāyan — meditating; devam — the Supreme Lord; adhārayat — worshiped.
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.