From Sahih muslim Chapter no 1:
Hadith No: 4 Narrated/Authority of Abu Huraira
One day, the Messenger of Allah appeared in public. A man approached him and asked, “Prophet of Allah, what is Iman?” The Holy Prophet replied, “Iman means affirming your faith in Allah, His angels, His Books, His meeting, His Messengers, and the Resurrection hereafter.” The man then asked, “Messenger of Allah, what does al-Islam signify?” The Holy Prophet answered, “Al-Islam signifies worshipping Allah alone, not associating anything with Him, establishing obligatory prayer, paying Zakat, and observing the fast of Ramadan.” The man continued, “Messenger of Allah, what does al-Ihsan imply?” The Holy Prophet explained, “Al-Ihsan means worshipping Allah as if you are seeing Him. And if you cannot see Him, then pray with the mindset that He is seeing you.” The man asked another question, “Messenger of Allah, when will the hour of Doom come?” The Holy Prophet replied, “No one knows when the hour of Doom will come, not even the one who is asked about it. But I can tell you some signs of its arrival, such as when a slave girl gives birth to her master, when the naked and barefooted become the chiefs of the people, and when shepherds of black camels boast about their extravagant buildings. Doom is one of the five unseen happenings that only Allah knows.” Then the Holy Prophet recited a verse emphasizing that only Allah has knowledge of the hour, the rain, what is in the wombs, and what people will earn tomorrow or where they will die. After the conversation, the man left. The Messenger of Allah then asked his companions to bring the man back. However, when they went to find him, he had disappeared. The Messenger of Allah explained that the man was Gabriel, who had come to teach the people about their religion.