Hadith 5712
Mishkat al-Masabih
Events of the Day of Resurrection and the Beginning of Creation · The Beginning of Creation, and Mention of the Prophets - Section 1
Arabic
وَعَنْ أَبِي هُرَيْرَةَ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِنَّمَا سُمِّيَ الْخَضِرُ لِأَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ فَإِذَا هِيَ تَهْتَزُّ من خَلْفِه خضراء» . رَوَاهُ البُخَارِيّ
English translation
Abu Huraira reported the Prophet (ﷺ) as saying, "The reason why al Khidr was so called was because he sat on some white waste land and it sprouted green (khadra') behind him." Bukhari transmitted it.