Musnad Abu Ya'la al-Mawsili 150:21

Narrated to us Ishaq, narrated to us Ishaq bin Sulayman, from al-Mughirah bin Ziyad, from ‘Ata, from ‘Aishah, who said: The Messenger of Allah (peace be upon him) said: "Whoever persists in twelve rak'ahs of the Sunnah, other than the obligatory prayer, Allah will build for him a house in Paradise: four before the Dhuhr, two after the Dhuhr, two after the ‘Isha, and two before the Fajr."