“Once the Iqamah has been called, there should be no prayer but the obligatory one.” Another chain with similar wording.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا أُقِيمَتِ الصَّلاَةُ فَلاَ صَلاَةَ إِلاَّ الْمَكْتُوبَةُ حَدَّثَنَا مَحْمُودُ بْنُ غَيْلاَنَ حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ أَنْبَأَنَا حَمَّادُ بْنُ زَيْدٍ عَنْ أَيُّوبَ عَنْ عَمْرِو بْنِ دِينَارٍ عَنْ عَطَاءِ بْنِ يَسَارٍ عَنْ أَبِي هُرَيْرَةَ عَنِ النَّبِيِّ ﷺ بِمِثْلِهِ  

“Which of your two prayers did you intend to be counted (i.e., accepted)?”  

أَنَّ رَسُولَ اللَّهِ ﷺ رَأَى رَجُلاً يُصَلِّي الرَّكْعَتَيْنِ قَبْلَ صَلاَةِ الْغَدَاةِ وَهُوَ فِي الصَّلاَةِ فَلَمَّا صَلَّى قَالَ لَهُ بِأَىِّ صَلاَتَيْكَ اعْتَدَدْتَ؟  

“The Prophet ﷺ passed by a man who was praying when the Iqamah for Subh prayer had been called, and he said something to him, I do not know what he said. When he finished, we surrounded the man and asked him: ‘What did the Messenger of Allah ﷺ say to you?’ He said: ‘He said to me: “Soon one of you will pray Fajr with four Rak’ah.’”  

مَرَّ النَّبِيُّ ﷺ بِرَجُلٍ وَقَدْ أُقِيمَتْ صَلاَةُ الصُّبْحِ وَهُوَ يُصَلِّي فَكَلَّمَهُ بِشَىْءٍ لاَ أَدْرِي مَا هُوَ فَلَمَّا انْصَرَفَ أَحَطْنَا بِهِ نَقُولُ لَهُ مَاذَا قَالَ لَكَ رَسُولُ اللَّهِ ﷺ قَالَ قَالَ لِي يُوشِكُ أَحَدُكُمْ أَنْ يُصَلِّيَ الْفَجْرَ أَرْبعًا