It is the wife’s right to take from her husband’s
money what is sufficient for her needs and the needs of her children
according to customary usage.
She may do so without his permission if he does not provide for her and her children sufficiently.
The evidence for this is that Hind, the wife of Abû Sufyân, came to the
Prophet (peace be upon him) and complained, saying: “O Messenger of
Allah! Abû Sufyân is a stingy man who does not provide for me what is
sufficient for me and my children. Is there any sin upon me if I take
from his wealth without his knowledge?”
The Prophet (peace be upon him) replied: “Take from his wealth what
suffices you and your children according to customary usage.” [Sahîh al-Bukhârî and Sahîh Muslim]
And Allah knows best.
(http://en.islamtoday.net/node/1847)