Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"We asked 'Ali about the voluntary (prayer) of Allah's Messenger (saw) during the day. He said: 'You will not be able.' We said: 'Inform us of it, we will do what we can of it?' So he said: 'When he prayed the Fajr he would delay praying any more. When the sun appeared over there (west) – like it appears here, meaning in the direction of the east, about the amount for the 'Asr prayer from there, meaning in the direction of the west, meaning before the Maghrib – he would stand and perform two Rakah* then he would delay praying until the sun appeared over there (west), meaning in the direction of the east, about the amount of the Zuhr prayer from there, then he would stand and perform four. And, four before the Zuhr when the sun passed the zenith, and two Rakah after it, and, four before the 'Asr, separating between every two Rakah with Taslim** upon the angels that are close (to Allah), the Prophets, and those who follow them among the Muslims and the believers.'" 'Ali said: "That is sixteen Rak'ah of voluntary prayer which Allah's Messenger (saw) performed during the day. And there are very few who offer them regularly." Sahih