Does a husband incur a sin if he does not oblige his wife to wear Hijaab, acting upon the Islamic principle that there is no compulsion in religion? It should be taken into account that she (the wife) regularly performs the prayer.

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

This husband incurs a huge sin for his stance. Allaah Almighty says (what means):

•        {O you who have believed, protect yourselves and your families from a Fire ….} [Quran 66: 6]

•        {Men are in charge of women by [right of] what Allaah has given one over the other ….} [Quran 4: 34]

Since a husband is responsible for his wife, he is not allowed to let her be seen without the Sharee'ah-approved Hijab. 

Allaah Knows best.