ENOfficial
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.
Source: sunnah.com
Other translations
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
sunnah.com