Can one begin a Tamattu` Hajj on the eighth of Thu'l-Hijjah, or must it begin before the eighth of Thu'l-Hijjah? What is the proof for this?
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A Tamattu' Hajj means to perform both Hajj and ‘Umrah on the same journey provided that the person would end his Ihraam (ritual consecration) for ‘Umrah before entering the Ihraam for Hajj. If he joins Hajj with ‘Umrah before performing Tawaaf (circumambulation of the Ka'bah), then his Hajj is considered as Qiraan and not Tamattu'. This condition is agreed upon by the scholars. If the pilgrim is able to perform ‘Umrah on the eighth day, ends his Ihraam by shortening his hair and then enters Ihraam for Hajj from where he is and goes to Mina, then this is alright. It is also permissible for the pilgrim to head directly to ‘Arafah on the ninth day without having gone to Mina on the eighth because staying in Mina on the eighth is only a recommended deed (and not one of the obligations of Hajj).
Al-Baaji said in Sharh Al-Muwatta’, "Maalik was reported to have said that a person who performs ‘Umrah on the day of Tarwiyah (i.e. the eighth of Thu'l-Hijjah) should not shave his head. Rather, he should shorten his hair and then perform Hajj." Shaykh Abu Bakr said, "Maalik believed that it was better to shorten his hair so as to have some hair remaining which he could then shave on the day of An-Nahr.”
Allah Knows best.