Imam Ahmad recorded that Abu Hurayrah, may Allah be pleased with him, said that the Prophet said concerning Al-Khidr؛

«إِنَّمَا سُمِّيَ خَضِرًا لِأَنَّهُ جَلَسَ عَلَى فَرْوَةٍ بَيْضَاءَ، فَإِذَا هِيَ تَهْتَزُّ مِنْ تَحْتِهِ خَضْرَاء»

(He was called Al-Khidr because he sat on a barren Farwah that turned white, then it turned green (Khadra') beneath him.) Imam Ahmad also recorded this from `Abdur-Razzaq. It was also recorded in Sahih Al-Bukhari from Hammam from Abu Hurayrah that the Messenger of Allah said,

«إِنَّمَا سُمِّي الْخَضِرَ لِأَنَّهُ جَلَسَ عَلَى فَرْوَةٍ، فَإِذَا هِيَ تَهْتَزُّ مِنْ تَحْتِهِ خَضْرَاء»

(He was called Al-Khidr because he sat on a barren Farwah and it turned green (Khadra') beneath him.) The meaning of Farwah here is a patch of withered vegetation. This was the view of `Abdur-Razzaq. It was also said that it means the face of the earth.

﴿ذَلِكَ تَأْوِيلُ مَا لَمْ تَسْطِـع عَّلَيْهِ صَبْراً﴾

(That is the interpretation of those (things) over which you could not be patient.) meaning, `this is the explanation of the things which you could not put up with or be patient with until I took the initiative of explaining them to you.' When he explained them and made them clear and solved the confusion, he used a milder form of the verb,

﴿تَسْطِـع﴾

(you could) When the matter was still confusing and very difficult, a more intensive form was used,

﴿سَأُنَبِّئُكَ بِتَأْوِيلِ مَا لَمْ تَسْتَطِع عَّلَيْهِ صَبْراً﴾

(I will tell you the interpretation of (those) things over which you were unable to be patient with) ﴿18:78﴾. The intensity of the verbal form used reflects the intensity of the confusion felt. This is like the Ayah:

﴿فَمَا اسْطَـعُواْ أَن يَظْهَرُوهُ﴾

(So they (Ya`juj and Ma`juj) were not able to scale it) ﴿18:97﴾ which means ascending to its highest point,

﴿وَمَا اسْتَطَـعُواْ لَهُ نَقْبًا﴾

(nor are they able to dig through it) ﴿18:97﴾ which is more difficult than the former. The intensity of the verbal form used reflects the difficulty of the action, which has to do with the subtleties of meaning. And Allah knows best. If one were to ask, what happened to the boy-servant of Musa who appears at the beginning of the story but then is not mentioned The answer is that the objective of the story is what happened between Musa and Al-Khidr. Musa's boy-servant was with him, following him. It is clearly mentioned in the Sahih Hadiths referred to above that he was Yusha` bin Nun, who was the one who became the leader of the Children of Israel after Musa, peace be upon him.

﴿وَيَسْأَلُونَكَ عَن ذِى الْقَرْنَيْنِ قُلْ سَأَتْلُواْ عَلَيْكُم مِّنْهُ ذِكْراً - إِنَّا مَكَّنَّا لَهُ فِى الاٌّرْضِ وَآتَيْنَـهُ مِن كُلِّ شَىْءٍ سَبَباً ﴾

(83. And they ask you about Dhul-Qarnayn. Say: "I shall recite to you something of his story.'') (84. Verily, We established him in the earth, and We gave him the means of everything.)