Narrated ‘Abdullah bin Umar (radiallaahu `anhu):
A person stood up and asked, “O Allah’s Apostle (sallallaahu `alayhi wasallam)! What clothes may be worn in the state of Ihram?” The Prophet (sallallaahu `alayhi wasallam) replied, “Do not wear a shirt or trousers, or any headgear (e.g. a turban), or a hooded cloak; but if somebody has no shoes he can wear leather stockings provided they are cut short off the ankles, and also, do not wear anything perfumed with Wars or saffron, and the Muhrima (a woman in the state of Ihram) should not cover her face, or wear gloves.”
Ihraam for men has been discussed in various posts before. As for women, they’re allowed to wear anything (that must cover their `awrah properly), except for perfume. And they must not cover their faces/hands except in case of necessity.
Women must uncover their faces and hands when they enter ihraam for Hajj or ‘Umrah. At this time, they are forbidden to wear niqaab and gloves, because the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The woman who is in ihraam must not wear niqaab or gloves.”
If a woman needs to cover her face because men are passing close by her, or she is beautiful and is sure that men are looking at her, she should drop a part of head covering over her face, because of the hadeeth of ‘Aa’ishah in which she said, “Riders were passing by us, and we were in ihraam with the Messenger of Allaah (peace and blessings of Allaah be upon him), so when they came near, each of us would lower her jilbaab over her face, and when they went away we would uncover our faces again.”
Al-Juzayri said, reporting from them: “A woman may cover her face for a necessary reason, such as non-mahram men passing close by her, and the fact that (the cloth) will touch her face does not matter. This is to make it easy and alleviate hardship.” (Al-Fiqh ‘ala’l-Madhaahib al-Arba’ah, 1/645).