The Prophet ﷺ never missed four rakʿat before the Zuhr prayer and two rakʿat before the Fajr prayer.  

أَنَّ النَّبِيَّ ﷺ كَانَ لاَ يَدَعُ أَرْبَعًا قَبْلَ الظُّهْرِ وَرَكْعَتَيْنِ قَبْلَ الْغَدَاةِ  

The Prophet ﷺ was never more regular and particular in offering any Nawafil than the two rakʿat (Sunna) of the Fajr prayer.  

لَمْ يَكُنِ النَّبِيُّ ﷺ عَلَى شَىْءٍ مِنَ النَّوَافِلِ أَشَدَّ مِنْهُ تَعَاهُدًا عَلَى رَكْعَتَىِ الْفَجْرِ  

The two rak'ahs at dawn are better than this world and what it contains.  

عَنِ النَّبِيِّ ﷺ قَالَ رَكْعَتَا الْفَجْرِ خَيْرٌ مِنَ الدُّنْيَا وَمَا فِيهَا  

Ziyadah al-Kindi reported on the authority of Bilal that he (Bilal) came to the Messenger of Allah ﷺ to inform him about the dawn prayer. Aisha kept Bilal engaged in a matter which she asked him till the day was bright and it became fairly light. Bilal then stood up and called him to prayer and called him repeatedly. The Messenger of Allah ﷺ did not yet come out. When he came out, he led the people in prayer and he (Bilal) informed him that Aisha had kept him engaged in a matter which she asked him till it became fairly light; hence he became late in reaching him (in time). He (Bilal) said: Messenger of Allah, the dawn became fairly bright. He said: If the dawn became brighter than it is now, I would pray them (the two rak'ahs of the sunnah prayer), offer them well and in a more beautiful manner.  

أَنَّهُ حَدَّثَهُ أَنَّهُ أَتَى رَسُولَ اللَّهِ ﷺ لِيُؤْذِنَهُ بِصَلاَةِ الْغَدَاةِ فَشَغَلَتْ عَائِشَةُ ؓ بِلاَلاً بِأَمْرٍ سَأَلَتْهُ عَنْهُ حَتَّى فَضَحَهُ الصُّبْحُ فَأَصْبَحَ جِدًّا قَالَ فَقَامَ بِلاَلٌ فَآذَنَهُ بِالصَّلاَةِ وَتَابَعَ أَذَانَهُ فَلَمْ يَخْرُجْ رَسُولُ اللَّهِ ﷺ فَلَمَّا خَرَجَ صَلَّى بِالنَّاسِ وَأَخْبَرَهُ أَنَّ عَائِشَةَ شَغَلَتْهُ بِأَمْرٍ سَأَلَتْهُ عَنْهُ حَتَّى أَصْبَحَ جِدًّا وَأَنَّهُ أَبْطَأَ عَلَيْهِ بِالْخُرُوجِ فَقَالَ إِنِّي كُنْتُ رَكَعْتُ رَكْعَتَىِ الْفَجْرِ فَقَالَ يَا رَسُولَ اللَّهِ إِنَّكَ أَصْبَحْتَ جِدًّا قَالَ لَوْ أَصْبَحْتُ أَكْثَرَ مِمَّا أَصْبَحْتُ لَرَكَعْتُهُمَا وَأَحْسَنْتُهُمَا وَأَجْمَلْتُهُمَا