By the fifth month, Mahārāja Dhruva, the son of the King, had controlled his breathing so perfectly that he was able to stand on only one leg, just as a column stands, without motion, and concentrate his mind fully on the Parabrahman.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पञ्चमे मास्यनुप्राप्ते जितश्वासो नृपात्मजः । ध्यायन् ब्रह्म पदैकेन तस्थौ स्थाणुरिवाचलः
pañcame māsyanuprāpte jitaśvāso nṛpātmajaḥ | dhyāyan brahma padaikena tasthau sthāṇurivācalaḥ
SB 4.8.76