Abu Dawud Book 2, Hadith Number 1245.

Chapter : The detailed injunctions about the supererogatory prayer and the rakahs of the sunan prayer.

Narated By Umm Habibah : The Prophet (PBUH) said: If anyone prays in the day and a night twelve rak’ahs voluntarily (supererogatory prayer), a house will be built for him in Paradise on account of these (rak’ahs).