Musnad Ahmad 788:223

Sulayman ibn Harb narrated to us, he said: Hammad ibn Zayd narrated to us, from Harun Abu Ishaq al-Kufi from Hamdan, from Abu Burdah ibn Abi Musa, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever prays twelve rak'ahs in a day and a night, besides the obligatory prayer, a house will be built for him in Paradise."