The Messenger of Allah ﷺ used to observe four rak'ahs in the forenoon prayer and he sometimes observed more as Allah pleased. (Using translation from Muslim 719c)   

كَانَ رَسُولُ اللهِ ﷺ «يُصَلِّي الضُّحَى أَرْبَعًا وَيَزِيدُ مَا شَاءَ اللهُ» خَالَفَهَا عُرْوَةُ  

I have never seen the Messenger of Allah ﷺ observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah ﷺ abandoned any act which he in fact loved to do, it was out of fear that if the people practised it constantly, it might become obligatory for them. (Using translation from Muslim 718)  

«مَا رَأَيْتُ رَسُولَ اللهِ ﷺ يُصَلِّي سُبْحَةَ الضُّحَى قَطُّ وَإِنِّي لَأُسَبِّحُهَا وَإِنْ كَانَ رَسُولُ اللهِ ﷺ لَيَدَعَ الْعَمَلَ وَهُوَ يُحِبُّ أَنْ يَعْمَلَهُ خَشْيَةَ أَنْ يَعْمَلَ بِهِ النَّاسُ فَيُفْرَضُ عَلَيْهِمْ»  

I aksed 'A'isha whether the Messenger of Allah ﷺ used to observe the forenoon prayer. She said: No, except when he came back from a journey. (Using translation from Muslim 717b)   

سَأَلْتُ عَائِشَةَ أَكَانَ رَسُولُ اللهِ ﷺ يُصَلِّي الضُّحَى؟ قَالَتْ «لَا إِلَّا أَنْ يَجِيءَ مِنْ مَغْيَبِهِ»  

[Machine] I stayed with Aisha, and when morning came, she got up and performed ablution, then she entered a room of hers and closed the door. I said, "O Mother of the believers, I have only come to you at this hour." She said, "Come in." So I entered and she got up and performed eight units of prayer. I do not know if her standing in prayer was longer or her bowing or prostration. Then she turned to me and hit my thigh, and said, "O Rumaitha, I saw the Messenger of Allah praying this prayer, and if my parents threatened me to leave it, I would not have left it." [265] And from Muhammad bin Salamah from Ibn Wahb from Amr bin al-Harith from Bukair bin al-Ashajj from Ya'qub bin al-Ashajj from al-Qa'qa bin Hakim that Rumaitha bint Hakim narrated to him, saying that she went to Aisha and mentioned that to her while standing.  

أَصْبَحْتُ عِنْدَ عَائِشَةَ فَلَمَّا أَصْبَحْنَا قَامَتْ فَاغْتَسَلَتْ ثُمَّ دَخَلَتْ بَيْتًا لَهَا فَأَجَافَتِ الْبَابَ قُلْتُ يَا أُمَّ الْمُؤْمِنِينَ مَا أَصْبَحْتُ عِنْدَكَ إِلَّا لِهَذِهِ السَّاعَةِ قَالَتْ فَادْخُلِي قَالَتْ فَدَخَلْتُ فَقَامَتْ فَصَلَّتْ ثَمَانِيَ رَكَعَاتٍ لَا أَدْرِي أَقِيَامُهُنَّ أَطْوَلُ أَمْ رُكُوعَهُنَّ أَمْ سُجُودُهُنَّ؟ ثُمَّ الْتَفَتَتْ إِلَيَّ فَضَرَبَتْ فَخِذِي فَقَالَتْ يَا رُمَيْثَةُ رَأَيْتُ رَسُولَ اللهِ ﷺ يُصَلِّيَهَا وَلَوْ نُشِرَ لِي أَبَوَايَ عَلَى تَرْكِهَا مَا تَرَكْتُهَا [267] 485 وَعَنْ مُحَمَّدِ بْنِ سَلَمَةَ عَنِ ابْنِ وَهْبٍ عَنْ عَمْرِو بْنِ الْحَارِثِ عَنْ بُكَيْرِ بْنِ الْأَشَجِّ عَنْ يَعْقُوبَ بْنِ الْأَشَجِّ عَنِ الْقَعْقَاعِ بْنِ حَكِيمٍ أَنَّ رُمَيْثَةَ بِنْتَ حَكِيمٍ حَدَّثَتْهُ أَنَّهَا أَتَتْ عَائِشَةَ فَذَكَرَهُ مَوْقُوفاً