Mahabharata 1.94.22

1.94.22

tāṃ dṛṣṭvā cintayāmāsa śaṃtanuḥ puruṣarṣabhaḥ |
syandate kiṃ nviyaṃ nādya saricchreṣṭhā yathā purā || 22 ||

"One day, while pursuing along the banks of the Ganges a deer that he had struck with his arrow, king Santanu observed that the river had become shallow. On observing this, that bull among men, viz., Santanu, began to reflect upon this strange phenomenon. He mentally asked why that first of rivers ran out so quickly as before. And while seeking for a cause, the illustrious monarch beheld that a youth of great comeliness, well-built and amiable person, like Indra himself, had, by his keen celestial weapon, checked the flow of the river. And the king, beholding this extraordinary feat of the river Ganga having been checked in her course near where that youth stood, became very much surprised. This youth was no other than Santanu's son himself. But as Santanu had seen his son only once a few moments after his birth, he had not sufficient recollection to identify that infant with the youth before his eyes. The youth, however, seeing his father, knew him at once, but instead of disclosing himself, he clouded the king's perception by his celestial powers of illusion and disappeared in his very sight.