Srimad-Bhagavatam: Canto 5 - Chapter 1 - Verse 10

Sanskrit:

यत्राश्रमपदान्युभयतोनाभिभिर्दृषच्चक्रैश्चक्रनदी नाम सरित्प्रवरा सर्वत: पवित्रीकरोति ॥ १० ॥

ITRANS:

yatrāśrama-padāny ubhayato nābhibhir dṛṣac-cakraiś cakra-nadī nāma sarit-pravarā sarvataḥ pavitrī-karoti.

Translation:

In Pulaha-āśrama is the Gaṇḍakī River, which is the best of all rivers. The śālagrāma-śilā, the marble pebbles, purify all those places. On each and every marble pebble, up and down, circles like navels are visible.

Purport:

Śālagrāma-śilā refers to pebbles that appear like stones with circles marked up and down. These are available in the river known as Gaṇḍakī-nadī. Wherever the waters of this river flow, the place becomes immediately sanctified.