The Quran

وَإِن فاتَكُم شَيءٌ مِن أَزواجِكُم إِلَى الكُفّارِ فَعاقَبتُم فَآتُوا الَّذينَ ذَهَبَت أَزواجُهُم مِثلَ ما أَنفَقوا ۚ وَاتَّقُوا اللَّهَ الَّذي أَنتُم بِهِ مُؤمِنونَ

Muhammad Asad

And if any of your wives should go over to the deniers of the truth, and you are thus afflicted in turn,¹ then give unto those whose wives have gone away the equivalent of what they had spent [on their wives by way of dower],² and remain conscious of God, in whom you believe!

Lit., "and you are thus taking your turn", i.e., like the unbelievers whose wives have gone over to the Muslims and renounced their erstwhile faith.
Since, as a rule, the unbelievers cannot really be expected to indemnify a husband thus deserted, the Muslim community as a whole is bound to undertake this obligation. As a matter of fact, there were only six such cases of apostasy in the lifetime of the Prophet (all of them before the conquest of Mecca in 8 H.); and in each case the Muslim husband was awarded by the communal treasury, on orders of the Prophet, the equivalent of the dower originally paid by him - (Baghawi and Zamakhshari).