What is the ruling on the a wife's action of taking from her extravagant husband's money, without his knowledge, to save it for the future out of fear of need?

If the husband is extravagant and imprudent in his financial dealings, the wife is permitted to bring his case before the Sharia court to have him interdicted. The wife is not allowed to interdict him herself; recourse to the judiciary is necessary to regulate the matter according to the strictures of Islamic law. The wife can advise him and remind him of the Quranic verses that forbid extravagance and waste, such as the Almighty’s saying: (And give the kinsman his due, and the poor and the traveler. And do not spend wastefully Indeed, the wasteful are brothers of the devils, and ever has Satan been to his Lord ungrateful) [Al-Isra: 26-27], and His saying, the Almighty: (Indeed, He likes not the extravagant) [Al-An'am: 141]. She can also seek help from his relatives to advise him, and supplicate for his guidance and rectitude.