The Prophet ﷺ said, "Al-Khadir was named so because he sat over a barren white land, it turned green with plantation after (his sitting over it."  

عَنِ النَّبِيِّ ﷺ قَالَ إِنَّمَا سُمِّيَ الْخَضِرُ أَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ فَإِذَا هِيَ تَهْتَزُّ مِنْ خَلْفِهِ خَضْرَاءَ  

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." (Using translation from Tirmidhī 3151)   

قَالَ رَسُولُ اللهِ ﷺ لَمْ يُسَمَّ خَضِرًا إِلَّا أَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ فَإِذَا هِيَ تَهْتَزُّ خَضْرَاءَ  

[Machine] From the Prophet ﷺ about Al-Khidr, he said: Al-Khidr was named "Al-Khidr" because he sat on a white quilt, and whenever he moved, it moved green.  

عَنِ النَّبِيِّ ﷺ فِي الْخَضِرِ قَالَ إِنَّمَا سُمِّيَ خَضِرًا أَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ فَإِذَا هِيَ تَحْتَهُ هِيَ تَحْتَهُ تَهْتَزُّ خَضْرَاءَ  

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." (Using translation from Tirmidhī 3151)  

قَالَ رَسُولُ اللَّهِ ﷺ «إِنَّمَا سُمَيَّ الْخَضِرُ خَضِرًا لِأَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ فَإِذَا هِيَ تَهْتَزُّ تَحْتَهُ خَضْرَاءَ»  

"إِنَّمَا سُمِّى الخَضِرُ خَضِرًا: لأَنَّه جَلَس عَلَى فَرْوَة بيضاءَ فإذَا هِى تَهْتَزُّ تَحْتَه خضراءَ ".

(كلُّ نبات يابس مجتمع فهو فروة).