Abu Huraira reported the Prophet as saying, "The reason why al Khidr was so called was because he sat on some white waste land and it sprouted green (khadra') behind him." Bukhari transmitted it.
Reported by Al-Bukhari (No. 3402), At-Tirmidhi (No. 3151) and Ahmad (No. 7765). Cited in Mishkat al-Masabih (No. 5712).
Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Al-Khidr was named so because he sat over a barren white land, it turned green with plantation after his sitting over it."