SB 4.8.76

pañcame māsy anuprāpte
 jita-śvāso nṛpātmajaḥ
dhyāyan brahma padaikena
 tasthau sthāṇur ivācalaḥ
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.
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.