The sage goes by various names: Satyatapas, Satyavrata, Utathya etc.
The full story is narrated in the Devī Bhāgavata, Skandha 3, Ch. XI.
23. Once on a time, a hunter named Nisâda, exceedingly clever in hunting, came accidentally with bows and arms in his hands, while
hunting a deer in that wide forest. He looked like a second God of
Death (Yama) and seemed to be very cruel.
24. That savage mountaineer, drawing his bow so as to touch the ear, pierced a boar with his sharp arrows. The boar, being very much
terrified, fled with enormous rapidity to the Muni Satyavrata.
37. O Jamadagni! Thus placed between the horns of a religious dilemma what shall I do now so as to meet both the ends -- to save the life of
the boar, to do the welfare, as well as not to speak untruth.”
38. When Satyavrata saw the boar wounded by the arrow of the hunter, he, moved with pity, uttered the seed mantra of the Goddess of
Learning; and now that most auspicious Goddess, on account of his
uttering Her seed mantram, was very pleased and gave him the
knowledge, difficult to be attained otherwise.
39. The door of all his knowledge opened out at once, and he became at once instantly the seer, the poet like the ancient Muni Vâlmikî.
40. Then that religiously disposed, merciful Brâhman, aiming at Truth, addressed that hunter before him with bows in his arms, thus :--
41. That force which sees (as witness) never speaks; and that force which speaks, never sees. O hunter! Why are you asking me repeatedly,
impelled by your own selfish desire?
42. The hunter, the killer of the animals, on hearing this was disappointed in the matter of finding out the boar and went back to
43. That Brâhmin turned out a poet like Varuna and he became celebrated as Satyavrata, the speaker of truth, in all the worlds.
44. He began to recite the Satyavrata mantram duly, and, by its influence, became a Pundit, rivalled by none in this world.
The verse uttered by Satyatapas to confuse the hunter is:
या पश्यति न सा ब्रूते या ब्रूते सा न पश्यति ।
अहो व्याध स्वकार्याथिन्कि पृच्छसि पुनः पुनः ॥
yā paśyati na sā brūte yā brūte sā na paśyati ।
aho vyādha svakāryāthinki pṛcchasi punaḥ punaḥ ॥
He who sees does not speak. He who speaks, does not see.
Selfish hunter, whom do you ask again and again?
In the Puranic Encyclopedia, you can read the brief history of how the brāhmaṇa boy named Utathya was cursed and how he transformed himself into Satyatapas.