Tawus (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) set out from Madinah without naming either Hajj or Umrah, and he kept awaiting the decision from Allah, until, while he (peace and blessings be upon him) was between Safa and Marwah, the decision was revealed to him. He (peace and blessings be upon him) ordered his companions that whoever had entered ihram for Hajj and had no sacrificial animal with him should make it an Umrah, and he said: "Had I known beforehand about this matter what I have come to know afterwards, I would not have brought the sacrificial animal. But I have matted my hair and brought my sacrificial animal, so there is no place of release for me other than the place of release of my sacrificial animal." Then Sayyiduna Suraqah bin Malik (may Allah be pleased with him) stood up and said: "O Messenger of Allah (peace and blessings be upon him), give us a decisive ruling, as though we were born today: is this Umrah of ours for this year only, or forever?" He (peace and blessings be upon him) said: "Rather, forever. Umrah has entered into Hajj until the Day of Resurrection." Sayyiduna Ali (may Allah be pleased with him) came from Yemen, and the Prophet (peace and blessings be upon him) asked him: "What talbiyah did you call?" He replied: I called the same talbiyah as the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8825