When the prayer commences then there is no prayer (valid), but the obligatory prayer. This hadith has been narrated by Warqa' with the same chain of transmitters.  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا أُقِيمَتِ الصَّلاَةُ فَلاَ صَلاَةَ إِلاَّ الْمَكْتُوبَةُ وَحَدَّثَنِيهِ مُحَمَّدُ بْنُ حَاتِمٍ وَابْنُ رَافِعٍ قَالاَ حَدَّثَنَا شَبَابَةُ حَدَّثَنِي وَرْقَاءُ بِهَذَا الإِسْنَادِ  

When the prayer commences, there is no prayer but the obligatory one.  

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِذَا أُقِيمَتِ الصَّلاَةُ فَلاَ صَلاَةَ إِلاَّ الْمَكْتُوبَةُ  

A hadith like this has been reported by Ishaq with the same chain of transmitters.  

أَخْبَرَنَا زَكَرِيَّاءُ بْنُ إِسْحَاقَ بِهَذَا الإِسْنَادِ مِثْلَهُ  

I then met 'Amr (the other narrator) and he narrated it to me, but it was not transmitted directly from the Messenger of Allah ﷺ.  

عَنِ النَّبِيِّ ﷺ بِمِثْلِهِ قَالَ حَمَّادٌ ثُمَّ لَقِيتُ عَمْرًا فَحَدَّثَنِي بِهِ وَلَمْ يَرْفَعْهُ  

The Messenger of Allah ﷺ happened to pass by a person who was busy in praying while the (Fard of the) dawn prayer had commenced. He said something to him, which we do not know what it was. When we turned back we surrounded him and said: What is it that the Messenger of Allah ﷺ said to you? He replied: He (the Holy Prophet) had said to me that he perceived as if one of them was about to observe four (rak'ahs) of the dawn prayer. Qa'nabi reported that 'Abdullah b. Malik b. Buhaina narrated it on the authority of his father. (Abu'l-Husain Muslim said): His assertion that he has narrated this hadith on the authority of his father is not correct.  

أَنَّ رَسُولَ اللَّهِ ﷺ مَرَّ بِرَجُلٍ يُصَلِّي وَقَدْ أُقِيمَتْ صَلاَةُ الصُّبْحِ فَكَلَّمَهُ بِشَىْءٍ لاَ نَدْرِي مَا هُوَ فَلَمَّا انْصَرَفْنَا أَحَطْنَا نَقُولُ مَاذَا قَالَ لَكَ رَسُولُ اللَّهِ ﷺ قَالَ قَالَ لِي يُوشِكُ أَنْ يُصَلِّيَ أَحَدُكُمُ الصُّبْحَ أَرْبَعًا قَالَ الْقَعْنَبِيُّ عَبْدُ اللَّهِ بْنُ مَالِكٍ ابْنُ بُحَيْنَةَ عَنْ أَبِيهِ قَالَ أَبُو الْحُسَيْنِ مُسْلِمٌ وَقَوْلُهُ عَنْ أَبِيهِ فِي هَذَا الْحَدِيثِ خَطَأٌ  

The dawn prayer had commenced when the Messen- ger of Allah ﷺ saw a person observing prayer, whereas the Mu'adhdhin had pronounced the Iqama. Upon this he (the Holy Prophet) remarked: Do you say four (rak'ahs) of Fard in the dawn prayer?  

أُقِيمَتْ صَلاَةُ الصُّبْحِ فَرَأَى رَسُولُ اللَّهِ ﷺ رَجُلاً يُصَلِّي وَالْمُؤَذِّنُ يُقِيمُ فَقَالَ أَتُصَلِّي الصُّبْحَ أَرْبَعًا  

A person entered the mosque, while the Messenger of Allah ﷺ was leading the dawn prayer. He observed two rak'ahs in a corner of the mosque, and then joined the Messenger of Allah ﷺ in prayer. When the Messenger of Allah ﷺ had pronounced salutations (he had concluded the prayer), he said: O, so and so, which one out of these two prayers did you count (as your Fard prayer), the one that you observed alone or the prayer that you observed with us?  

دَخَلَ رَجُلٌ الْمَسْجِدَ وَرَسُولُ اللَّهِ ﷺ فِي صَلاَةِ الْغَدَاةِ فَصَلَّى رَكْعَتَيْنِ فِي جَانِبِ الْمَسْجِدِ ثُمَّ دَخَلَ مَعَ رَسُولِ اللَّهِ ﷺ فَلَمَّا سَلَّمَ رَسُولُ اللَّهِ ﷺ قَالَ يَا فُلاَنُ بِأَىِّ الصَّلاَتَيْنِ اعْتَدَدْتَ أَبِصَلاَتِكَ وَحْدَكَ أَمْ بِصَلاَتِكَ مَعَنَا