It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he said: "I had a dispute with a man of Banu Fazarah about the person whom Sayyiduna Musa (peace be upon him) followed." I said: "He was al-Khadir (peace be upon him)," while the Fazari said: "He was someone else." Meanwhile Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) passed by us. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: So I called him and asked: "Did you hear the Messenger of Allah (peace and blessings be upon him) mention the person whom Sayyiduna Musa (peace be upon him) followed?" He said: "Yes, I heard the Messenger of Allah (peace and blessings be upon him) say: 'Once Musa (peace be upon him) was sitting in a gathering of the Children of Israel when a man asked him: Is there anyone more knowledgeable about Allah than you? He replied: Whom would I consider to be so? (meaning: in my view there is no one). At this Allah Almighty reproached him, because he had not referred the knowledge to Allah, and Allah revealed: Indeed there is - My servant al-Khadir (is ahead of you in knowledge). Sayyiduna Musa (peace be upon him) asked for the way to reach him, so Allah Almighty made the fish a sign for him: at the place where he found the fish missing (there he would meet him). Then there happened to him what Allah Almighty has related (in the Qur'an).'"
Imam al-Tabarani (may Allah have mercy on him) said: "Only Muslim bin Khalid (may Allah have mercy on him) and Abdullah bin Maymun al-Qaddah (may Allah have mercy on him) narrated this hadith from Ja'far bin Muhammad (may Allah have mercy on him)."
Hadith Referenceالمعجم الأوسط / باب الميم / 6217
Hadith Takhrij«أخرجه البخاري فى «صحيحه» برقم: 74، 78، 122، 2267، 2728، 3278، 3400، 3401، 4725، 4726، 4727، 6672، 7478، ومسلم فى «صحيحه» برقم: 2380، 2661، وابن حبان فى «صحيحه» برقم: 102، 988، 6220، 6221، والحاكم فى «المستدرك» برقم: 3414، 4117، والنسائي فى «السنن الكبرى» برقم: 5813، 11196، 11244، 11245، 11246، 11247، 11248، وأبو داود فى «سننه» برقم: 3984، 4706، 4705، 4707، والترمذي فى «جامعه» برقم: 3149، 3150، 3385، وأحمد فى «مسنده» برقم: 21497، 21505، 21506، 21507، 21508، 21509، 21510، 21511، 21502، 21504، 21515، 21519، 21520، 21516، والطيالسي فى «مسنده» برقم: 540، والحميدي فى «مسنده» برقم: 375، وعبد بن حميد فى «مسنده» برقم: 169، وابن أبي شيبة فى «مصنفه» برقم: 29836، والطحاوي فى «شرح مشكل الآثار» برقم: 3125، 4895، والطبراني فى «الكبير» برقم: 45، 46»