The Messenger of Allah ﷺas saying: When the iqamah is pronounced for prayer, no prayer is valid except the obligatory prayer. (Using translation from Abū Dāʾūd 1266)  

قَالَ رَسُولُ اللَّهِ ﷺ «إِذَا أُقِيمَتِ الصَّلَاةُ فَلَا صَلَاةَ إِلَّا الْمَكْتُوبَةُ»  

This tradition has also been narrated on the authority of ‘A’ishah through a different chain. (Using translation from Abū Dāʾūd 99)  

عَنِ النَّبِيِّ ﷺ نَحْوَهُ  

[Machine] The translation is: The prayer was established, and the Prophet ﷺ saw a man praying two units of prayer. When the Prophet ﷺ finished his prayer, the people approached him and said: "Do you pray four units for the Fajr prayer?"  

أُقِيمَتِ الصَّلَاةُ فَرَأَى النَّبِيُّ ﷺ رَجُلًا يُصَلِّي الرَّكْعَتَيْنِ فَلَمَّا قَضَى النَّبِيُّ ﷺ صَلَاتَهُ لَاثَ بِهِ النَّاسُ فَقَالَ لَهُ النَّبِيُّ ﷺ «أَتُصَلِّي الصُّبْحَ أَرْبَعًا؟»  

[Machine] Regarding the Prophet ﷺ , he said, "When the prayer is established, there is no prayer except the obligatory prayer." Abu Muhammad said, "If he is in his house, then the house is easier."  

عَنِ النَّبِيِّ ﷺ قَالَ «إِذَا أُقِيمَتِ الصَّلَاةُ فَلَا صَلَاةَ إِلَّا الْمَكْتُوبَةُ» قَالَ أَبُو مُحَمَّدٍ إِذَا كَانَ فِي بَيْتِهِ فَالْبَيْتُ أَهْوَنُ