Kuvera water

And beholding Rama the descendant of Ikshwaku's race, quite at his ease, Vibhishana, O son
of Pritha, joining his hands; told him these words, 'O chastiser of foes, at the command of the king of the
Guhyakas, a Guhyaka hath come from the White mountains, bringing with him his water! O great king, this water is a present to thee from Kuvera, so that all creatures that are invisible may, O chastiser of foes, become visible to thee!

This water laved over the eyes will make every invisible creature visible to thee, as also to any other
person to whom thou mayst give it!' --Saying--So be it, --Rama took that sacred water, and sanctified his own eyes therewith. And the
high-minded Lakshmana also did the same. And Sugriva and Jambuvan, and Hanuman and Angada, and
Mainda and Dwivida, and Nila and many other foremost of the monkeys, laved their eyes with that
water. And thereupon it exactly happened as Vibhishana had said, for, O Yudhishthira, soon did the eyes
of all these became capable of beholding things that could not be seen by the unassisted eye!

Vana Parva: Section 287