"Water is for Water." That is, water for peforming Ghusl is obligatory when the ejaculation of water (semen) occurs.
Sunan an-Nasa'i Book 1, Hadith 200
← Previous Hadith "I asked the Messenger of Allah (ﷺ) about a woman who has a wet dream and he said: 'If she sees water, let her perform Ghusl.'"

Sunan an-Nasa'i Book 1, Hadith 199 Next Hadith → "The Messenger of Allah (ﷺ) said: 'The man's water is thick and white, and the woman's water is thin and yellow. Whichever of them comes first, the child will resemble (that parent).'"

Sunan an-Nasa'i Book 1, Hadith 201