Musnad Abu Ya'la al-Mawsili 3:357

Zuhayr narrated to us, Waki' narrated to us, from Sufyan, from Abu Ishaq, from 'Asim, from 'Ali, who said: The Prophet, peace and blessings of Allah be upon him, used to pray two rak'ahs after every prescribed prayer, except for the Fajr and the 'Asr.