Is the niqab obligatory, or is the hijab what is obligatory? And are there any verses or hadiths that prove the niqab is obligatory, given that one of the conditions for a woman in ihram during Hajj is to uncover her face and hands?

A woman's covering her face with the niqab is obligatory, based on evidence from the Qur’an and Sunnah, except during Hajj. For a woman, her ihram (state of ritual consecration) is in her face and hands. She is forbidden from covering her face unless she is in the presence of non-mahram men. If she is in their presence, it becomes obligatory for her to cover her face with something that is not tailored to the face and not clinging to it, as Aisha (may Allah be pleased with her) did, when she would let down her jilbab over her face if riders passed by her while she was in ihram. Al-Qadi Abu Bakr ibn al-Arabi mentioned that covering the face is obligatory except during Hajj, in which case she should let down something from her head covering over her face, not clinging to it, and she should turn away from men and they should turn away from her.