It was narrated from Abu Ayyub that the Prophet (ﷺ) said:
"Water is for Water." [1][1] That is, water for peforming Ghusl is obligatory when the ejaculation of water (semen)occurs. (IT)
English reference (IT): Vol. 1, Book 1, Hadith 199 (IT)
Arabic reference (IT): Book 1, Hadith 200 (IT)