Mustadrak al-Hakim — Hadith 8850

It is narrated from Sayyiduna Hudhayfah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Islam will wear out and fade away just as the embroidery of a garment fades, until it will no longer be known what fasting, charity and the sacrifice of Hajj and Umrah are. The Book of Allah, the Mighty and Majestic, will be taken away in a single night, so that not a single verse of it will remain on the earth. Some groups of people will remain, among them old men and old women, who will say: We found our forefathers upon this word, 'La ilaha illallah' (there is no god but Allah), so we too say it." At this Silah asked him: When they will not even know about fasting, charity and sacrifice, of what use will this word 'La ilaha illallah' be to them? Sayyiduna Hudhayfah (may Allah be pleased with him) turned away from him. Silah asked again, until he asked a third time, and then he turned towards Silah and said: O Silah! This word will save them from the Fire; it will protect them from the Fire; it will deliver them from the Fire of Hell.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.