Sahih Muslim 711
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is disliked to start a voluntary prayer after the Mu’adhdhin has started to say Iqamah for prayer, whether that is a regular sunnah, such as the sunnah of Subh or Zuhr, or anything else, and regardless of whether he knows that he will catch up with the rak`ah with the Imam or not
Ibn Buhaina reported:
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ ، عَنْ حَفْصِ بْنِ عَاصِمٍ ، عَنِ ابْنِ بُحَيْنَةَ ، قَالَ أُقِيمَتْ صَلاَةُ الصُّبْحِ فَرَأَى رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً يُصَلِّي وَالْمُؤَذِّنُ يُقِيمُ فَقَالَ " أَتُصَلِّي الصُّبْحَ أَرْبَعًا " .
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?