(a) Tariq bin Shihab says: I came to Abdullah (may Allah be pleased with him) and said that a woman among us wants to combine Umrah with her Hajj. Abdullah (may Allah be pleased with him) said: Allah says: "The Hajj is in well-known months" [Surah al-Baqarah: 197], and I consider these alone to be the months of Hajj.
(b) Zayd bin Suhan and Salman bin Rabi'ah disliked it, and its permissibility is stated by Umar bin al-Khattab (may Allah be pleased with him). The reason for the dislike is the narration we have transmitted from Sayyiduna Umar (may Allah be pleased with him). It is reported from Abdullah bin Mas'ud (may Allah be pleased with him): These are two rites, and I like that each of them should have its own dishevelment and its own journey. The permissibility of tamattu', qiran and ifrad is established from the Messenger of Allah (peace and blessings be upon him). What concerns the Hajj ifrad of the Messenger of Allah (peace and blessings be upon him) has passed earlier, and the disagreement in the early period over the dislike of tamattu' and qiran has also passed. Performing Hajj separately from Umrah (ifrad) is more virtuous. Wallahu a'lam (And Allah knows best).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8887