Chandogya Upanishad 2.4.1

2.4.1

sarvāsv apsu pañcavidhaṃ sāmopāsīta \
megho yat saṃplavate sa (S )kāraḥ \
yad varṣati sa prastāvaḥ \
yāḥ prācyaḥ syandante sa udgīthaḥ \
yāḥ pratīcyaḥ sa pratihāraḥ \
samudro nidhanam \\204.1\

1. In all waters one should reverence a fivefold Sāman. When a cloud gathers—that is a Hiṅkāra. When it rains—that is a Prastāva.

Those [waters] which flow to the east—they are an Udgītha. Those which flow to the west—they are a Pratihāra. The ocean is a Nidhana.