Mu'jam Kabir Tabarani 7499 — the Messenger of Allah ﷺ said

Sayyiduna Abu Umamah (may Allah be pleased with him) narrated that the Messenger of Allah (peace and blessings be upon him) said to his Companions: "Shall I not tell you about al-Khidr (peace be upon him)?" They said: Of course, O Messenger of Allah. He (peace and blessings be upon him) said: One day al-Khidr (peace be upon him) was walking in the market of the Children of Israel when a mukatab (a slave who has contracted to buy his freedom for a sum of money) saw him and said: Give me charity, may Allah bless you. Al-Khidr said: I believe in Allah; whatever matter Allah wills comes to be. I have nothing to give you. The poor man said: I ask you by Allah, why do you not give me charity? For I saw the mark of goodness in your face and hoped for blessing from you. Al-Khidr said: I believe in Allah. I have nothing to give you, except that you take me and sell me. The poor man said: Would that be right? Al-Khidr said: Yes, I speak the truth. You have asked me by something very great; I will not disappoint one who asks by my Lord. Sell me. So he took him to the market and sold him for four hundred dirhams. He stayed with the buyer for a long time without being put to any work, so al-Khidr said to him: You only bought me seeking some good from me, so tell me of some work to do. He said: I dislike causing you hardship; you are an old, weak man. Al-Khidr said: It will be no hardship for me. He said: Then get up and move these stones. Those stones could not be moved in a day by fewer than six men. The man went out for some need of his, then came back and saw that al-Khidr had moved all the stones in a single hour. He said: You have done well and excellently, and you have accomplished what I did not expect of you. Then the man had to go on a journey, and he said: I consider you trustworthy, so look after my family well in my absence. Al-Khidr said: Tell me of some work to do. He said: I dislike causing you hardship. Al-Khidr said: It will be no hardship for me. He said: Then make some bricks for my house until I return to you. The man went on his journey, then returned and saw that al-Khidr had built his building strongly. He said: I ask you by Allah, what is your way and what is your story? Al-Khidr said: You have asked me by Allah, and it was asking by Allah that cast me into slavery. Al-Khidr said: I will tell you who I am. I am that al-Khidr of whom you have heard. A poor man asked me for charity and I had nothing to give him; he asked me by Allah, so I gave him possession of my neck and he sold me. And I tell you that whoever is asked by Allah and turns the asker away while he is able to give, will be made to stand on the Day of Resurrection with only his skin, with no flesh and no bone, the empty skin rattling. At this the man said: I believe in Allah. O Prophet of Allah, I have caused you hardship without knowing it. Al-Khidr said: There is no harm; you did well and kept faith. The man said: May my father and mother be ransomed for you, O Prophet of Allah! Judge concerning my family and my wealth as Allah shows you, or I give you the choice that I set you free. Al-Khidr said: I would like you to set me free so that I may worship my Lord. So he set him free, and al-Khidr said: All praise is for Allah, who cast me into slavery and then delivered me from it.