Tawoos says that the Prophet ﷺ went out and he was not mentioning Hajj or Umrah by name, rather he was awaiting revelation. Tawoos said that when he ﷺ was making rounds between Safa and Marwah, revelation descended upon him. The Prophet ﷺ then ordered his companions, may Allah be pleased with them, that whoever among them had entered into Ihram for Hajj and did not have a sacrificial animal with him, should make it Umrah instead. Then he said: "If I had known beforehand what I came to know later, I would not have brought the sacrificial animal, but I have already stuck to my intention and brought the sacrificial animal with me, so it is not permissible for me to become lawful (exit Ihram) without offering the sacrifice." Then Suraqah bin Malik, may Allah be pleased with him, stood up towards the Prophet ﷺ and said: "O Messenger of Allah! Will you make a ruling about us as if we are a new people born today? Is this act of converting Hajj into Umrah specific to this year of ours, or is it permitted forever?" The Messenger of Allah ﷺ said: "It is permitted forever; Umrah has been included in the (days of) Hajj until the Day of Judgment." He said that Ali, may Allah be pleased with him, entered (Makkah) from Yemen, so the Prophet ﷺ asked: "With what intention did you enter into Ihram?" One of Ibn Tawoos or Ibrahim bin Maisarah said: "I entered into Ihram and recited the Talbiyah like the Ihram of the Prophet ﷺ," and the other said: "I recited the Talbiyah for Hajj like the Hajj of the Prophet ﷺ."
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 805
Hadith Takhrijاسناده ضعيف لإرساله اخرجه البيهقي : 6/5 ۔ وفى المعرفة السنن والآثار له (2683)۔