The Prophet ﷺ said about Khidr, "He was named Khidr only because he sat upon a patch of white, barren ground, and suddenly beneath him it stirred and turned green."
حدثنا يحيى بن آدم حدثنا ابن مبارك عن معمر عن همام بن منبه
عن النبي ﷺ في الخضر قال إنما سمي خضرا أنه جلس على فروة بيضاء فإذا هي تحته تهتز خضراء
Its isnad is sahih. It is found in Jami al-Masanid 7:390, taken from this very place. Bukhari related it in 6:308 (Fath) from Muhammad bin Saeed Asbahani, from Abdullah bin Mubarak, with this same isnad. It will come again in the Sahifah of Hammam bin Munabbih at 8211. Bukhari did not relate it through the route of the Sahifah. This hadith is one of his unique reports, which Muslim did not relate in his Sahih, as the Hafiz stated explicitly in Fath al-Bari 6:381. Suyuti marked it in Jami Saghir with the symbol for hadiths agreed upon by both, meaning that he claimed Muslim also related it. This is an error on his part.