Mishkat al-Masabih 28 Events of the Day of Resurrection and the Beginning of Creation كتاب أحوال القيامة وبدء الخلق 182

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 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.