Sayyidina Khidr, Abul `Abbas , is the one whom Allah mentioned in the Holy Qur'an [18:65f.] as the servant of Allah who met with the Prophet Musa and taught him by means of a series of practical lessons about the Will of God and His Actions.
Irfan Omar in "Khidr in the Islamic Tradition":
Khidr is one of the four prophets whom the Islamic tradition recognizes as being ‘alive’ or ‘immortal’. The other three being Idris (Enoch), Ilyas (Elias), and ‘Isa (Jesus). Khidr is immortal because he drank from the water of life. There are some who have asserted, however, that this Khidr is the same person as Elijah. He is also identified with St. George.
In Islamic folk literature, one finds a variety of names and titles associated with Khidr. Some say Khidr is a title; others have called it an ephithet. He has been equated with St. George, identified as the Muslim “version of Elijah” and also referred to as the eternal wanderer. Scholars have also called and characterized him as a ‘saint’, prophet-saint, mysterious prophet-guide and so on.
The story of Khidr finds its source in the Qur’ān, chapter 18 (Sūrat Kahf) verses 60-82,
Then they found one of Our votaries whom We had blessed and given knowledge from Our Presence.
These verses primarily deal with an allegorical story relating Moses’ journey in search for truth. Full of symbolism, the Qur’ānic story introduces the mysterious figure of Khidr, who symbolizes “the utmost depth of mystic insight accessible to man. While Khidr is not mentioned in the Qur’ān by name on the basis of Hadīth literature, the mysterious person with whom Moses’ meets as mentioned in 18:65, and who is called in the Qur’ān as “one of Our votaries”, is no other than the ‘eternal’ Khidr.
Irfan Omar, "The Muslim World", Vol. LXXXIII, No. 3-4, July - October,
1993. Duncan Black MacDonald Center for the Study of Islam and
 Annemarie Schimmel, Mystical Dimensions of Islam, (Chapel Hill: University of North Carolina Press. 1975), 202.
 “Muslim version of Elijah” George K. Anderson. The Legend of the Wandering Jew (Providence: Brown University Press. 1965), 409; Exhaustive material on Khidr’s resemblance with Elijah is presented in Friedlaenders “Khidr” in the Encyclopedia of Religion and Ethics (New York: Charles Scribner’s Sons, 1915), 693-95.
 Peter L. Wilson, “The Green Man: The Trickster Figure in Sufism”, in Gnosis Magazine 1991, 23.
 On Rodwell, see W.M. Thackston Jr.. The Tales of the Prophets of al-Kisai /(Boston: Twayne Publishers, 1978), xxiv.
 Alexander H. Krappe. The Science of Folklore (New York: Barnes and Noble Inc., 1930), 103.
 However, he refers to the Wandering Jew as Ahasver. See Haim Schwarzbaum. Biblical and Extra-Biblical Legends, 17.
 Schwarzbaum mentions several references to it in his excellent work Biblical and Extra-Biblical Legends. For instance Tabari’s Tatsir (Cairo: 1373). v. III, 28-29; and Tha’labi’s ‘Ara’is al-Majalis (Cairo: 1324), 126. It is the intermingling with the wild beast which links Khidr to the prophet Jeremiah. I. Friedlaender has further linked it back to the legend of the ‘Wandering Jew.’ See Schwarzbaum, 167-68.
 Historical identity here does not mean determining the actual person of Khidr in history but as it is related in the “divergent sources” such as prophetology, folklore, etc. See Henry Corbin. Creative Imagination in the Sufism of Ibn ‘Arabi (Princeton: Princeton University Press. 1969). 55.
 See Wensinck “al-Khadir” in The Encyclopedia of Islam. 861-865