Al-Khidr was named so because he sat over a barren white land, it turned green with plantation after his sitting over it

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."

The Prophet (may Allah's peace and blessings be upon him) says that Al-Khidr got this name because once he sat on a barren piece of land that had no plantation, and it turned into a greenery. Al-Khidr is mentioned in the story of Mūsa in Sūrat al-Kahf. Scholars differed whether he was a prophet or a righteous ally of Allah. The sound opinion is that he is a prophet.