SB 4.8.76

pañcame māsy anuprāpte
 jita-śvāso nṛpātmajaḥ
dhyāyan brahma padaikena
 tasthau sthāṇur ivācalaḥ
Synonyms: 
pañcame — in the fifth; māsi — in the month; anuprāpte — being situated; jita-śvāsaḥ — and still controlling the breathing; nṛpa-ātmajaḥ — the son of the King; dhyāyan — meditating; brahma — the Supreme Personality of Godhead; padā ekena — with one leg; tasthau — stood; sthāṇuḥ — just like a column; iva — like; acalaḥ — without movement.
Translation: 
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.