The Great Epic of the Bhārata dynasty 3.85.9

3.85.9

sā ca puṇyajalā yatra phalgunāmā mahānadī |
bahumūlaphalā cāpi kauśikī bharatarṣabha |
viśvāmitro'bhyagādyatra brāhmaṇatvaṃ tapodhanaḥ || 9 ||

It is for this that the ancients say that one should wish for many sons, so that even one among them may visit Gaya, celebrate the horse-sacrifice or give away a nila bull, and thereby deliver ten generations of his race up and down. There, O monarch, is a great river, and spot called Gayasira. In Gayasira is a banian, which is called by the Brahmanas the Eternal banian, for the food that is offered there to the Pitris becometh eternal, O exalted one! The great river that floweth by the place is known by the name of Phalgu, and its waters are all sacred. And, O bull among the Bharatas, there also, in that place, is the Kausiki, whose basin abounds in various fruit and roots, and where Viswamitra endued with wealth of asceticism acquired Brahmanahood. Towards that direction also is the sacred Ganga, on whose banks Bhagiratha celebrated many sacrifices with profuse gifts (to Brahmanas). They say that in the country of Panchala, there is a wood called Utpala, where Viswamitra of Kusika's race had performed sacrifices with his son, and where beholding the relics of Viswamitra's superhuman power, Rama, the son of Jamadagni, recited the praises of his ancestry. At Kamyaka, Kusika's son had quaffed the Soma juice with Indra. Then abandoning the Kshatriya order, he began to say, I am a Brahmana.' In that quarter, O hero is the sacred confluence of Ganga and Yamuna which is celebrated over the world. Holy and sin-destroying, that tirtha is much regarded by the Rishis.