Narrated Abu Hurairah:
that the Messenger of Allah (ﷺ) said: "He was called Al-Khadir because he sat on a barren Farwah and it turned green beneath him." (DE)
English reference (DE): Vol. 5, Book 44, Hadith 3151 (DE)
Arabic reference (DE): Book 47, Hadith 3444 (DE)