Musnad Ahmad 5:448

Waki’ and ‘Abd al-Rahman narrated to us, from Sufyan, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray two rak'ahs after every obligatory prayer, except for the Fajr and the ‘Asr. And ‘Abd al-Rahman said: "At the end of every prayer."