Musnad Ahmad 5:659

'Abdullah narrated to us, Ishaq bin Isma'il narrated to us, Waki' narrated to us, Sufyan narrated to us, from Abu Ishaq, from 'Asim bin Damrah al-Saluli, 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."