SB 4.8.43

snātvānusavanaṁ tasmin
 kālindyāḥ salile śive
kṛtvocitāni nivasann
 ātmanaḥ kalpitāsanaḥ
Word for word: 
snātvā — after taking bath; anusavanam — three times; tasmin — in that; kālindyāḥ — in the river Kālindī (the Yamunā); salile — in the water; śive — which is very auspicious; kṛtvā — performing; ucitāni — suitable; nivasan — sitting; ātmanaḥ — of the self; kalpita-āsanaḥ — having prepared a sitting place.
Translation: 
Nārada Muni instructed: My dear boy, in the waters of the Yamunā River, which is known as Kālindī, you should take three baths daily because the water is very auspicious, sacred and clear. After bathing, you should perform the necessary regulative principles for aṣṭāṅga-yoga and then sit down on your āsana [sitting place] in a calm and quiet position.
Purport: 

It appears from this statement that Dhruva Mahārāja had already been instructed how to practice the eightfold yoga system, which is known as aṣṭāṅga-yoga. This system is explained in our Bhagavad-gītā As It Is, in the chapter entitled “Dhyāna-yoga.” It is understood that in aṣṭāṅga-yoga one practices settling the mind and then concentrating it on the form of Lord Viṣṇu, as will be described in the following verses. It is clearly stated here that aṣṭāṅga-yoga is not a bodily gymnastic exercise, but a practice to concentrate the mind on the form of Viṣṇu. Before sitting on his āsana, which is also described in Bhagavad-gītā, one has to cleanse himself very nicely in clear or sacred water thrice daily. The water of the Yamunā is naturally very clear and pure, and thus if anyone bathes there three times, undoubtedly he will be very greatly purified externally. Nārada Muni, therefore, instructed Dhruva Mahārāja to go to the bank of the Yamunā and thus become externally purified. This is part of the gradual process of practicing mystic yoga.