Musnad Ahmad 1159:17

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, from an-Nu'man ibn Salim, from 'Amr ibn Aws, from 'Anbasah ibn Abi Sufyan, from his sister Umm Habibah, the wife of the Prophet, may Allah bless him and grant him peace, that she heard the Prophet, may Allah bless him and grant him peace, saying: "There is no Muslim servant who prays to Allah, the Almighty and Majestic, every day twelve voluntary rak'ahs, other than the obligatory ones, except that a house is built for him in Paradise, or Allah, the Almighty and Majestic, builds for him by them a house in Paradise." Umm Habibah said: "I have not ceased praying them since." And 'Amr said: "I have not ceased praying them since." And an-Nu'man said: "The same as that."