Musnad Ahmad 650
It was narrated that `Asim bin Damrah said:
I asked `Ali (رضي الله عنه) about the nafl prayers of the Prophet (ﷺ)  during the day. He said: You are not able for that. We said: Tell us and we will do as much of it as we can, He said: When the Messenger of Allah (ﷺ) prayed Fajr, he would wait until the sun would rise from there, meaning in the east, as high as it is at the time of `Asr there, meaning in the west. Then he would get up and pray two rak`ahs. Then he would wait until the sun rose as high there, meaning in the east, as it is at the time of Zuhr there, meaning in the west; then he would pray four rak`ahs, and four before Zuhr when the sun passed the meridian, and four afterwards, and four before ‘Asr, He would separate each two rak’ahs with salams upon the angels who are close to Allah, the Prophets, the believers and the Muslims who follow them. `Ali (رضي الله عنه) said: That is sixteen rak`ahs which the Prophet (ﷺ) offered as nafl prayers during the day, but there are very few who offer them regularly. Wakee’ narrated; my father said: Habeeb bin Abi Thabit said to Abu Ishaq when he narrated this to him: O Abu Ishaq, this hadeeth of yours is worth this mosque filled with gold.
حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا سُفْيَانُ، وَإِسْرَائِيلُ، وَأَبِي، عَنْ أَبِي إِسْحَاقَ، عَنْ عَاصِمِ بْنِ ضَمْرَةَ، قَالَ سَأَلْنَا عَلِيًّا رَضِيَ اللَّهُ عَنْهُ عَنْ تَطَوُّعِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِالنَّهَارِ فَقَالَ إِنَّكُمْ لَا تُطِيقُونَهُ قَالَ قُلْنَا أَخْبِرْنَا بِهِ نَأْخُذْ مِنْهُ مَا أَطَقْنَا قَالَ كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا صَلَّى الْفَجْرَ أَمْهَلَ حَتَّى إِذَا كَانَتْ الشَّمْسُ مِنْ هَا هُنَا يَعْنِي مِنْ قِبَلِ الْمَشْرِقِ مِقْدَارُهَا مِنْ صَلَاةِ الْعَصْرِ مِنْ هَاهُنَا مِنْ قِبَلِ الْمَغْرِبِ قَامَ فَصَلَّى رَكْعَتَيْنِ ثُمَّ يُمْهِلُ حَتَّى إِذَا كَانَتْ الشَّمْسُ مِنْ هَاهُنَا يَعْنِي مِنْ قِبَلِ الْمَشْرِقِ مِقْدَارُهَا مِنْ صَلَاةِ الظُّهْرِ مِنْ هَاهُنَا يَعْنِي مِنْ قِبَلِ الْمَغْرِبِ قَامَ فَصَلَّى أَرْبَعًا وَأَرْبَعًا قَبْلَ الظُّهْرِ إِذَا زَالَتْ الشَّمْسُ وَرَكْعَتَيْنِ بَعْدَهَا وَأَرْبَعًا قَبْلَ الْعَصْرِ يَفْصِلُ بَيْنَ كُلِّ رَكْعَتَيْنِ بِالتَّسْلِيمِ عَلَى الْمَلَائِكَةِ الْمُقَرَّبِينَ وَالنَّبِيِّينَ وَمَنْ تَبِعَهُمْ مِنْ الْمُؤْمِنِينَ وَالْمُسْلِمِينَ قَالَ قَالَ عَلِيٌّ رَضِيَ اللَّهُ عَنْهُ تِلْكَ سِتَّ عَشْرَةَ رَكْعَةً تَطَوُّعُ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِالنَّهَارِ وَقَلَّ مَنْ يُدَاوِمُ عَلَيْهَا حَدَّثَنَا وَكِيعٌ عَنْ أَبِيهِ قَالَ قَالَ حَبِيبُ بْنُ أَبِي ثَابِتٍ لَأَبِي إِسْحَاقَ حِينَ حَدَّثَهُ يَا أَبَا إِسْحَاقَ يَسْوَى حَدِيثُكَ هَذَا مِلْءَ مَسْجِدِكَ ذَهَبًا‏.‏