The Prophet ﷺ said, "When the prayer has been called to begin, no prayer is to be performed except the obligatory one."
عن النبي الله ﷺ قال: إذا أقيمت الصلاة فلا صلاة إلا المكتوبة.
Muslim narrated it in the Book of the Prayer of Travelers (710) from Ahmad bin Hanbal, and it is in his Musnad (9873). He said, Muhammad bin Jafar told us, Shuba told us, from Warqa, from Amr bin Dinar, from Ata bin Yasar, from Abu Hurairah, and he related it. Warqa is Ibn Umar al-Yashkuri, Abu Bishr al-Kufi, one of the narrators found in all six major collections. Some narrators transmitted it from Amr bin Dinar and made it stop with him, but the correct view is that it is raised to the Prophet, and most of the reliable narrators support this. See for more, Minna Kubra (2/305–306).This hadith has also been narrated from Ibn Umar, transmitted by Abdullah bin Marwan al-Dimashqi from Ibn Abu Dhib, from Ibn Umar. Tahawi narrated it by this route in his Mushkil (4132). Dhahabi discussed this in Mizan, in the entry on Abdullah bin Marwan, and said, "This wording actually belongs to Amr bin Dinar, from Ata bin Yasar, from Abu Hurairah, raised to the Prophet." This is because, as Ibn Hibban said about Abdullah bin Marwan, "He narrated from Ibn Abu Dhib, and Sulayman narrated from him. He attaches sahih texts to other chains. It is not permissible to rely on him as evidence."As for occupying oneself with the two rakahs of Fajr when the call to begin the prayer is given, see the hadiths on this in the comprehensive chapters on supererogatory prayers connected to the obligatory prayers, in the chapter on the dislike of occupying oneself with the two rakahs of Fajr when the prayer has been called to begin.