Mustadrak al-Hakim — Hadith 3441

It is narrated from Mus'ab bin Sa'd; he said: I was reciting before my father (Sayyiduna Sa'd bin Abi Waqqas, may Allah be pleased with him) until, when I reached this verse: "Say: Shall We inform you of those who are the greatest losers in respect of their deeds?" [Surah al-Kahf: 103], I said: "O my father, does this refer to the Khawarij?" He said: "No, my son. Recite the verse after it: 'Those are the ones who disbelieved in the signs of their Lord and in the meeting with Him, so their deeds have become worthless, and We will not assign to them any weight on the Day of Resurrection' [Surah al-Kahf: 105]. This refers to those among the Christians who strive hard in their religion (monks and the like). Their disbelief in the signs of their Lord was that they disbelieved in Muhammad (peace and blessings be upon him) and in the meeting with Allah, and they said that there will be no food or drink in Paradise. As for the Khawarij, they are the rebellious sinners about whom it is said: 'Those who break the covenant of Allah after its confirmation, and sever that which Allah has ordered to be joined, and cause corruption on earth. It is they who are the losers' [Surah al-Baqarah: 27]."
The chain of narration of this hadith is sahih (authentic), but the two Shaykhs (Bukhari and Muslim) did not record it!