Abu Huraira reported the Messenger of Allah (ﷺ) as saying: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
This hadith has been narrated by Abu Huraira with another chain of transmitters. Hammad (one of the narrators) said: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)
Abdullah b. Malik b. Buhaina reported: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
Ibn Buhaina reported: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?
Hadith 1651 — Sahih Muslim 6:81
حَدَّثَنَا أَبُو كَامِلٍ الْجَحْدَرِيُّ، حَدَّثَنَا حَمَّادٌ يَعْنِي ابْنَ زَيْدٍ، ح وَحَدَّثَنِي حَامِدُ بْنُ عُمَرَ الْبَكْرَاوِيُّ، حَدَّثَنَا عَبْدُ الْوَاحِدِ يَعْنِي ابْنَ زِيَادٍ، ح وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، كُلُّهُمْ عَنْ عَاصِمٍ، ح وَحَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، - وَاللَّفْظُ لَهُ - حَدَّثَنَا مَرْوَانُ بْنُ مُعَاوِيَةَ الْفَزَارِيُّ، عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ عَبْدِ اللَّهِ بْنِ سَرْجِسَ، قَالَ دَخَلَ رَجُلٌ الْمَسْجِدَ وَرَسُولُ اللَّهِ صلى الله عليه وسلم فِي صَلاَةِ الْغَدَاةِ فَصَلَّى رَكْعَتَيْنِ فِي جَانِبِ الْمَسْجِدِ ثُمَّ دَخَلَ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَلَمَّا سَلَّمَ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَ " يَا فُلاَنُ بِأَىِّ الصَّلاَتَيْنِ اعْتَدَدْتَ أَبِصَلاَتِكَ وَحْدَكَ أَمْ بِصَلاَتِكَ مَعَنَا " .
Abdullah b. Sarjis reported: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?
Abu Usaid reported that the Messenger of Allah (ﷺ) said:When any one of you enters the mosque, he should say:" O Allah! open for me the doors of Thy mercy" ; and when he steps out he should say: 'O Allah! I beg of Thee Thy Grace." (Imam Muslim said: I heard Yahya saying: I transcribed this hadith from the compilation of Sulaiman b. Bilal)