Mishkat al-Masabih · Book 28 · Hadith 5712
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
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.