I heard Allah's Apostle (ﷺ) as saying: I shall go to the Cistern before you and he who comes would drink and he who drinks would never feel thirsty, and there would come to me people whom I would know and who would know me. Then there would be intervention between me and them. Abu Hazim said that Nu'man b. Abu 'Ayyash heard it and I narrated to them this hadith, and said: Is it this that you heard Sahl saying? He said: Yes, and I bear witness to the fact that I heard it from Abu Sa'id Khudri also, but he made this addition that he (the Holy Prophet) would say: They are my followers, and it would be said to him: You do not know what they did after you and I will say to them: Woe to him who changes (his religion) after me.
The Book of Virtues - Sahih Muslim 2290, 2291a
This narration from Abu Sa'id al-Khudri (may Allah be pleased with him) concerns the Hawd al-Kawthar (the Cistern) which Allah has granted to the Prophet Muhammad (peace be upon him) as an honor and distinction on the Day of Judgment.
The Nature of the Cistern
The Hawd is a vast reservoir whose water is whiter than milk and sweeter than honey. Its drinking vessels are numerous like the stars, and whoever drinks from it once will never thirst again. This signifies the ultimate spiritual fulfillment for the believers who follow the Prophet's guidance.
Recognition and Separation
The Prophet will recognize his true followers, and they will recognize him. However, some who claimed to be from his Ummah will be prevented from reaching him. This intervention occurs due to their innovation (bid'ah) and deviation from the authentic Sunnah after the Prophet's death.
The Grave Warning
When the Prophet intercedes for those turned away, he will be informed: "You do not know what they introduced after you." The Prophet's response - "Woe to those who changed after me!" - serves as a severe warning against altering the religion through innovation or abandoning the authentic teachings he left behind.
Scholarly Commentary
Imam al-Nawawi explains this demonstrates the Prophet's concern for his Ummah and the gravity of introducing innovations. Ibn Hajar al-Asqalani notes this highlights the importance of adhering to the pure Sunnah and avoiding all newly-invented matters in religion. The scholars unanimously agree this refers to those who introduced heresies or abandoned the Prophetic way.