SB 5.8.1
sri-suka uvaca
ekada tu maha-nadyam krtabhiseka-naiyamikavasyako brahmaksaram abhigrnano muhurta-trayam udakanta upavivesa.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sri Sukadeva Gosvami continued: My dear King, one day, after finishing his morning duties — evacuating, urinating and bathing — Maharaja Bharata sat down on the bank of the river Gandaki for a few minutes and began chanting his mantra, beginning with omkara.