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. (JA)
English reference (JA): Vol. 1, Book 1, Hadith 199 (JA)
Arabic reference (JA): Book 1, Hadith 200 (JA)