Musnad Ahmad Hadith 1202

Some of the companions of `Ali (رضي الله عنه) said: Why don`t you tell us about the voluntary prayers that the Messenger of Allah (ﷺ) offered during the day? `Ali (رضي الله عنه) said: By Allah, you cannot do it. They said to him; Tell us about it and we will take whatever we are able to do... then he quoted the hadeeth at length. It was narrated from ‘Ali (رضي الله عنه) that He was asked about the (voluntary) prayers of the Messenger of Allah (ﷺ) during the day. He said: He used to pray sixteen rak`ahs, When the sun was as high there as it is at the time of ‘Asr there, he would pray two rak’ahs. When the sun was as high there as it is at the time of Zuhr there, he would pray two rak`ahs, He would pray four rak`ahs before Zuhr and two rak`ahs after Zuhr, and he would pray four rak`ahs before ʻAsr.