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." (ID)
English reference (ID): Vol. 5, Book 44, Hadith 3151 (ID)
Arabic reference (ID): Book 47, Hadith 3444 (ID)