SB 5.8.1

śrī-śuka uvāca
ekadā tu mahā-nadyāṁ kṛtābhiṣeka-naiyamikāvaśyako brahmākṣaram abhigṛṇāno muhūrta-trayam udakānta upaviveśa.
Synonyms: 
śrī-śukaḥ uvāca — Śrī Śukadeva Gosvāmī said; ekadā — once upon a time; tu — but; mahā-nadyām — in the great river known as Gaṇḍakī; kṛta-abhiṣeka-naiyamika-avaśyakaḥ — having taken a bath after finishing the daily external duties such as passing stool and urine and brushing the teeth; brahma-akṣaram — the praṇava-mantra (om); abhigṛṇānaḥ — chanting; muhūrta-trayam — for three minutes; udaka-ante — on the bank of the river; upaviveśa — he sat down.
Translation: 
Śrī Śukadeva Gosvāmī continued: My dear King, one day, after finishing his morning duties — evacuating, urinating and bathing — Mahārāja Bharata sat down on the bank of the river Gaṇḍakī for a few minutes and began chanting his mantra, beginning with oṁkāra.