Musnad Abu Ya'la al-Mawsili 3:313

Ubayd Allah narrated to us, Abd al-Rahman narrated to us, Sufyan narrated to us, from Abu Ishaq, from Asim ibn Damrah, from Ali: That the Messenger of Allah (may Allah bless him and grant him peace) used to pray two rak'ahs after every prescribed prayer, except for the Asr and the Subh (Fajr) prayers.