Musnad Ahmad 30:1109

'Abd al-Razzaq ibn Hammam narrated to us, Ma'mar narrated to us, from Hammam, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "He was only named Al-Khidr because he sat upon a white *farwah* (surface/vegetation), and it suddenly shook, becoming green." The *farwah* is white grass and what resembles it. 'Abdullah said: "I think this is an explanation from 'Abd al-Razzaq."