Ibn Maajah (4245) narrated from Thawbaan (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said:
“I certainly know people of my ummah who will come on the Day of Resurrection with good deeds like the mountains of Tihaamah, but Allaah will make them like scattered dust.” Thawbaan said: O Messenger of Allaah, describe them to us and tell us more, so that we will not become of them unknowingly. He said: “They are your brothers and from your race, worshipping at night as you do, but they will be people who, when they are alone, transgress the sacred limits of Allaah.”
This is a saheeh hadeeth; it was classed as such by al-Buwaysari in Misbaah al-Zujaajah (4/246) and al-Albaani in al-Saheehah. Al-Mundhiri said in al-Targheeb wa’l-Tarheeb (3/170): Its narrators are thiqaat (trustworthy).