Short answer
Yes. Covering the awrah is a condition of prayer, even if no one can see you. A woman must cover her hair and whole body except the face, hands and feet. If a quarter of a part such as the hair shows for the length of one pillar of prayer, the prayer is invalid in the Hanafi school.
Covering the awrah is fard for the validity of salah, alone or in congregation. Hanging hair is part of the awrah. If a quarter of one limb of awrah is uncovered for as long as it takes to say Subhan Allah three times, the prayer is invalid; less than a quarter is excused.
The four schools agree on this.
Explanation
Many sisters assume hijab is only about men seeing them. In prayer it is different: covering the awrah is one of the conditions of salah itself, like wudu (ablution) and facing the qiblah. It applies when you pray alone in your room just as it does in the masjid. The four schools agree on this.
What to cover in prayer, on the Hanafi view:
- All of the hair, including hair hanging below the ears.
- The neck, ears, arms and forearms up to the wrists.
- The legs and the rest of the body.
- The face, the hands up to the wrists and the feet may stay uncovered. See can a woman pray with her feet uncovered.
The cloth must be thick enough that hair and skin colour do not show through. A thin chiffon scarf that shows the hair underneath does not count as covering. Many women keep a simple prayer garment, a long khimar or a two-piece prayer set, by the prayer mat so it is quick to put on.
The Hanafi measure for slips: if a quarter of one part of the awrah, for example a quarter of the hair or of one forearm, becomes exposed and stays exposed for as long as one pillar of prayer, the prayer must be repeated. If you notice it and cover it straight away, the prayer is fine. For the full method see how women pray.
The evidence
Quran, Surah Al-A'raaf 7:31
۞ يَٰبَنِىٓ ءَادَمَ خُذُوا۟ زِينَتَكُمْ عِندَ كُلِّ مَسْجِدٍۢ وَكُلُوا۟ وَٱشْرَبُوا۟ وَلَا تُسْرِفُوٓا۟ ۚ إِنَّهُۥ لَا يُحِبُّ ٱلْمُسْرِفِينَ ﴿٣١﴾
O children of Adam, take your adornment at every masjid, and eat and drink, but be not excessive. Indeed, He likes not those who commit excess.
Commands covering and proper dress at every place and time of prayer.
Sunan Abi Dawud 641
Graded sahih... لا يقبل الله صلاة حائض إلا بخمار ...
"Allah does not accept the prayer of a woman who has reached puberty unless she wears a head covering."
Shows that a head covering is a condition of a woman's prayer, with no mention of who is watching.
Where the ruling is recorded
- Nur al-Idah with Maraqi al-Falah, Chapter on the conditions of prayer. The free woman's awrah in prayer and the quarter rule for exposure.
- al-Durr al-Mukhtar, Kitab al-Salah, chapter on the conditions of prayer. Hanging hair is awrah; exposure of a quarter for one rukn invalidates.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.