Ibn Shihab narrates that Urwah bin al-Zubayr said that Sayyidah Aishah (may Allah be pleased with her), the wife of the Prophet (peace and blessings be upon him), informed him about the tamattu' of the Messenger of Allah (peace and blessings be upon him), combining Umrah with Hajj, and the people's tamattu' along with him, just like what Salim bin Abdullah informed me from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) from the Messenger of Allah (peace and blessings be upon him). So I said to Salim bin Abdullah: Then why do you forbid me from tamattu', when the Messenger of Allah (peace and blessings be upon him) did it and the people did it with him? Salim said: Abdullah bin Umar (may Allah be pleased with them both) informed me that Umar bin al-Khattab (may Allah be pleased with him) said: "The most complete way for Umrah is that you keep it separate from the months of Hajj. The months of Hajj are well known: Shawwal, Dhu al-Qa'dah and Dhu al-Hijjah; so devote them purely to Hajj, and perform Umrah in the other months."
By this Sayyiduna Umar (may Allah be pleased with him) intended the completion of Umrah, because of the saying of Allah, the Mighty and Majestic: "And complete the Hajj and Umrah for Allah" [Surah al-Baqarah: 196]. That is because an Umrah in which a person performs tamattu' into Hajj is not complete unless he offers a sacrificial animal, or, if he does not find a sacrificial animal, fasts three days during Hajj and seven when he returns to his family; whereas an Umrah outside the months of Hajj is complete without a sacrificial animal or fasting. So by what he ordered, namely leaving tamattu' of Umrah into Hajj, Sayyiduna Umar (may Allah be pleased with him) intended the completion of the Umrah that Allah, the Mighty and Majestic, has commanded. Sayyiduna Umar (may Allah be pleased with him) also wanted the House to be visited twice every year, and he disliked that people should perform tamattu' of Umrah into Hajj and that this become their settled practice, so that they would come to the House only once a year. Then the rulers became strict about tamattu', until people thought that the rulers regarded it as forbidden. By my life, the rulers did not regard it as forbidden; rather, seeking reward, they followed what Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) had ordered in that matter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8874