Narrated Umm Habibah (may Allah be pleased with her):
The Prophet (peace and blessings of Allah be upon him) as saying: If anyone prays in a day and a night twelve rak’ahs voluntarily (supererogatory prayer), a house will be built from him in Paradise on account of these (rak’ahs).
Sunan Abi Dawud 1250, Book 5, Hadith 1