It wasnarrated that 'Asim bin Damrah As-Saluli said:
"We asked 'Aliabout the voluntary (prayer) of Allah's Messenger (ﷺ) during theday. He said: 'You will not be able.' We said: 'Inform us ofit, we will do what we can of it?' So he said: 'When he prayedthe Fajr he would delay praying any more. When the sun appeared overthere (west) – like it appears here, meaning in the direction ofthe east, about the amount for the 'Asr prayer from there, meaningin the direction of the west, meaning before the Maghrib – he wouldstand and perform two Rak'ah* then he would delay praying until thesun appeared over there (west), meaning in the direction of the east,about the amount of the Zuhr prayer from there, then he would standand perform four. And, four before the Zuhr when the sun passed thezenith, and two Rak'ah after it, and, four before the 'Asr,separating between every two Rak'ah with Taslim** upon the angelsthat are close (to Allah), the Prophets, and those who follow themamong the Muslims and the believers.'"'Ali said: "That is sixteen Rak'ah of voluntary prayer which Allah's Messenger (ﷺ) performed during the day. And there are very few who offer them regularly."Waki` said: "My father added: Habib bin Abu Thabit said: 'O Abu Ishaq, this mosque filled with gold would not be dearer to me than this Hadith of yours.'"
English reference: Vol. 1, Book 5, Hadith 1161
Arabic reference: Book 5, Hadith 1216