The definitive discovery of Moon water came from Chandrayaan-1 which carried with it a NASA-provided science instrument called the Moon Mineralogical Mapper—M3 for short—that observed how the surface absorbed infrared light. Using this data, M3 determined that previously suspected water molecules were ice inside the Moon’s polar craters [0].

However, the first direct evidence of water vapor near the Moon was obtained by the Apollo 14 in 1971 [0]. A series of bursts of water vapor ions were observed by the instrument mass spectrometer at the lunar surface near the Apollo 14 landing site.

[0]: https://www.planetary.org/articles/water-on-the-moon-guide

[1]: https://en.m.wikipedia.org/wiki/Lunar_water