Tamattu

It is narrated from Imam al-Hasan al-Basri (may Allah have mercy on him) that the Commander of the Believers, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), firmly intended to forbid Hajj al-Tamattu' (performing Umrah in the months of Hajj and leaving ihram). Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) stood up before him and said: "You do not have this authority, for the Book of Allah, the Mighty and Majestic, has been revealed (with its ruling) concerning it, and we ourselves performed Umrah in this way together with the Messenger of Allah (peace and blessings be upon him)." So upon hearing this, Sayyiduna Umar (may Allah be pleased with him) desisted from his intention.

Ibn Tawus narrates from his father that my father and Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) came into the presence of the Commander of the Believers, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), and asked him: "Will you not inform the people about the matter of this mut'ah (Hajj al-Tamattu')?" At this Sayyiduna Umar (may Allah be pleased with him) said: "Is there now anyone left who has not done it? As for me, I myself practise it."
In this narration, "mut'ah" means Hajj al-Tamattu', in which ihram is removed after performing Umrah in the months of Hajj, and then ihram for Hajj is entered on the same journey. The intent of Sayyiduna Umar (may Allah be pleased with him) was that the legitimacy of this practice had now become so common and well known that no ambiguity of any kind remained about it.

Sayyiduna Ibn Abbas (may Allah be pleased with them both) states: I heard Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) saying: "If I were to perform Umrah again and again and then intend Hajj, I would choose Hajj al-Tamattu'."
According to the Imams and commentators, this report makes clear that in the view of Sayyiduna Umar al-Faruq (may Allah be pleased with him), among the types of Hajj, Hajj al-Tamattu' (in which ihram is removed after performing Umrah in the months of Hajj, and then Hajj is performed on the same journey) was a preferred and superior practice, and his own inclination was towards this form of worship which the Messenger of Allah (peace and blessings be upon him) had taught his Companions.

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Commander of the Believers, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), said: "If people separate Hajj and Umrah (that is, perform these two acts of worship on separate journeys), and Umrah is performed outside the months of Hajj, on the other days of the year, then this way will be more suitable for the most complete performance of the Hajj and the Umrah of each one of you."

It is narrated from Nafi' (may Allah have mercy on him) that when Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) entered ihram (for Hajj) from Makkah, he would not perform the sa'y between Safa and Marwah until he returned from Mina (after performing the other rites of Hajj); that is, he would delay the sa'y until the Tawaf al-Ifadah.
From this juristic report the Imams have inferred that for one who enters the ihram of Hajj from Makkah (a mutamatti' or a resident of Makkah), the Sunnah is to perform the sa'y along with the Tawaf al-Ziyarah, and performing sa'y before departing for Mina is not required of him.

It is narrated from Hafsah (may Allah have mercy on her) that she stated: We entered ihram for Umrah in the blessed month of Ramadan, and then we reached Makkah in the month of Shawwal, while in those days there was a great crowd of people there. So we asked (about the legal nature of this act of worship of ours), and whomever we asked, he said that it is "Hajj al-Tamattu'".

It is narrated from Mujahid (may Allah have mercy on him) that Sayyiduna Abdullah bin al-Zubayr (may Allah be pleased with them both) said (addressing the people of Makkah): "Perform only Hajj al-Ifrad, and leave the saying of this blind man of yours (Ibn Abbas)." At this Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said (promptly): "In reality, it is you yourself whose heart Allah Almighty has deprived of insight (made blind). Just ask your mother (Sayyidah Asma', may Allah be pleased with her) to learn the legal status of this matter."

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he attained the blessing of Hajj al-Tamattu' (that is, removing ihram after performing Umrah on a single journey and then entering ihram for Hajj) in the company of the Messenger of Allah (peace and blessings be upon him).
Allah, Glorified and Exalted, knows best the reality of the matter.