The Messenger of Allah ﷺ saw a person praying after the congregational prayer at dawn was over. The Messenger of Allah ﷺ said: There are two rak'ahs of the dawn prayer (i.e. the prescribed rak'ahs). The man replied: I did not pray the two rak'ahs before the dawn prayer. Hence I offered them now. The Messenger of Allah ﷺ kept silent. (Using translation from Abū Dāʾūd 1267)   

رَأَى النَّبِيُّ ﷺ رَجُلًا يُصَلِّي بَعْدَ صَلَاةِ الصُّبْحِ رَكْعَتَيْنِ فَقَالَ رَسُولُ اللهِ ﷺ أَصَلَاةُ الصُّبْحِ مَرَّتَيْنِ؟ فَقَالَ الرَّجُلُ إِنِّي لَمْ أَكُنْ صَلَّيْتُ الرَّكْعَتَيْنِ اللَّتَيْنِ قَبْلَهُمَا فَصَلَّيْتُهُمَا الْآنَ قَالَ فَسَكَتَ رَسُولُ اللهِ ﷺ  

[Machine] He went out in the early morning and found the Prophet ﷺ praying in the early morning, and he had not performed his two units of Fajr prayer. So he prayed with the Prophet ﷺ , then when he finished the morning prayer, he performed his two units of Fajr prayer. The Prophet ﷺ passed by him and said, "What is this prayer?" He informed him, so the Prophet ﷺ remained silent and continued on without saying anything.  

خَرَجَ إِلَى الصُّبْحِ فَوَجَدَ النَّبِيَّ ﷺ فِي الصُّبْحِ وَلَمْ يَكُنْ رَكَعَ رَكْعَتَيِ الْفَجْرِ فَصَلَّى مَعَ النَّبِيِّ ﷺ ثُمَّ قَامَ حِينَ فَرَغَ مِنَ الصُّبْحِ فَرَكَعَ رَكْعَتَيِ الْفَجْرِ فَمَرَّ بِهِ النَّبِيُّ ﷺ فَقَالَ مَا هَذِهِ الصَّلَاةُ؟ فَأَخْبَرَهُ فَسَكَتَ النَّبِيُّ ﷺ وَمَضَى وَلَمْ يَقُلْ شَيْئًا