Musnad Ahmad 1273:2

Hushaym narrated to us, he said: Dawud ibn Abi Hind informed us, from al-Nu’man ibn Salim, from ‘Anbasah ibn Abi Sufyan, who said: Umm Habibah bint Abi Sufyan informed me that the Messenger of Allah (peace and blessings of Allah be upon him) used to say: "Whoever prays twelve voluntary rak'ahs in a day, other than the obligatory ones, a house will be built for him in Paradise."