Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.028B.

Ibn al-Munzhir says: “There is consensus among the scholars that a person who declares his intention to perform Hajj or ‘Umrah and puts on his ihram before reaching the miqat does enter the state of ihram. But is it undesirable to do so? Some say it is, in light of the saying of the Companions that the Prophet (peace be upon him) himself fixed Zhul Hulaifah as miqat for the people of Madinah which means that (an intending pilgrim) should make his ihram from these specified places, without any addition or deletion from the rule. But if addition is not prohibited, then at least the best course would be not to do it (i.e. not make ihram before reaching the fixed place).”