The Prophet ﷺ said, "Khidr was named so because he once sat upon a patch of white, withered ground, and behold, it began to shimmer green behind him."
عن النبي ﷺ قال: إنما سمي الخضر أنه جلس على فروة بيضاء، فإذا هي تهتز من خلفه خضراء.
Bukhari related it in the Hadiths of the Prophets (3402), from Muhammad bin Saeed Asbahani, who said, Ibn Mubarak informed us, from Mamar, from Hammam bin Munabbih, from Abu Hurairah, who mentioned it.Al-Farwah means the dry ground.