Musnad Ahmad 30:994

Yahya ibn Adam narrated to us, Ibn al-Mubarak narrated to us, from Ma'mar, from Hammam ibn Munabbih, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, regarding al-Khidr, he said: "He was only named al-Khidr because he sat upon a white patch of land, and suddenly it was beneath him, it was beneath him, shaking with green."