Asmā’ bint Abu Bakr (may Allah be pleased with her) reported: The Prophet (may Allah’s peace and blessings be upon him) said: "I will be on the Cistern so that I see who among you will come to me. But some people will be barred from reaching me. Thereupon, I will say: 'O Lord, they belong to me and my Ummah.' It will be said: 'Do you know what they did after you? By Allah, they quickly turned back upon their heels.'" Sahih/Authentic. - [Al-Bukhari and Muslim]
The Prophet (may Allah’s peace and blessings be upon him) informs in the Hadīth that he will be on the Cistern on the Day of Judgment and will see who among his followers will come there. Some of his Ummah will be detained nearby. So, he will say: O Lord, they belong to me and my Ummah. It will be said: Do you know what they did after you left them? By Allah, they kept turning back upon their heels and leaving their religion. They do not belong to you or your Ummah. This will happen to a small group of those who believed during his lifetime; they do not include the eminent Companions who fought the apostates, remained steadfast to Islam, and died as Muslims (may Allah be pleased with them).