Once during the lifetime of Messenger of Allah ﷺ we offered some animals as sacrifices. Some people slaughtered their sacrifices before the (Id) prayer, so when the Prophet ﷺ finished his prayer, he saw that they had slaughtered their sacrifices before the prayer. He said, "Whoever has slaughtered (his sacrifice) before the prayer, should slaughter (another sacrifice) in lieu of it; and whoever has not yet slaughtered it till we have prayed; should slaughter (it) by mentioning Allah's Name."  

ضَحَّيْنَا مَعَ رَسُولِ اللَّهِ ﷺ أُضْحِيَّةً ذَاتَ يَوْمٍ فَإِذَا أُنَاسٌ قَدْ ذَبَحُوا ضَحَايَاهُمْ قَبْلَ الصَّلاَةِ فَلَمَّا انْصَرَفَ رَآهُمُ النَّبِيُّ ﷺ أَنَّهُمْ قَدْ ذَبَحُوا قَبْلَ الصَّلاَةِ فَقَالَ مَنْ ذَبَحَ قَبْلَ الصَّلاَةِ فَلْيَذْبَحْ مَكَانَهَا أُخْرَى وَمَنْ كَانَ لَمْ يَذْبَحْ حَتَّى صَلَّيْنَا فَلْيَذْبَحْ عَلَى اسْمِ اللَّهِ  

[Machine] "He (referring to someone) prayed with the Messenger of Allah ﷺ on the day of Eid, and then the Messenger of Allah ﷺ left. Suddenly, he found meat and the sacrifices of Eid, so the Messenger of Allah ﷺ knew that they had been slaughtered before he prayed. So the Messenger of Allah ﷺ said, 'Whoever slaughtered before we prayed, let him slaughter another one in its place. And whoever did not slaughter until we prayed, let him slaughter in the name of Allah.'"  

أَنَّهُ صَلَّى مَعَ رَسُولِ اللهِ ﷺ يَوْمَ أَضْحَى فَانْصَرَفَ رَسُولُ اللهِ ﷺ فَإِذَا هُوَ بِاللَّحْمِ وَذَبَائِحِ الْأَضْحَى فَعَرَفَ رَسُولُ اللهِ ﷺ أَنَّهَا ذُبِحَتْ قَبْلَ أَنْ يُصَلِّيَ فَقَالَ رَسُولُ اللهِ ﷺ مَنْ كَانَ ذَبَحَ قَبْلَ أَنْ نُصَلِّيَ فَلْيَذْبَحْ مَكَانَهَا أُخْرَى وَمَنْ لَمْ يَكُنْ ذَبَحَ حَتَّى صَلَّيْنَا فَلْيَذْبَحْ بِاسْمِ اللهِ  

I was with Messenger of Allah ﷺ on the day of 'Id al-Adha. While he had not returned after having offered (the Id prayer) and finished it, he saw the flesh of the sacrificial animals which had been slaughtered before he had completed the prayer. Thereupon he (the Holy Prophet) said: One who slaughtered his sacrificial animal before his prayer or our prayer ('Id), he should slaughter another one in its stead, and he who did not slaughter, he should slaughter by reciting the name of Allah. (Using translation from Muslim 1960a)  

شَهِدْتُ الْأَضْحَى مَعَ رَسُولِ اللهِ ﷺ فَلَمْ يُعِدْ أَنْ صَلَّى وَفَرَغَ مِنْ صَلَاتِهِ إِذَا هُوَ يَرَى لَحْمَ أَضْحًى قَدْ ذُبِحَتْ قَبْلَ أَنْ يَفْرُغَ مِنْ صَلَاتِهِ فَقَالَ رَسُولُ اللهِ ﷺ «مَنْ كَانَ ذَبَحَ أُضْحِيَّتَهُ قَبْلَ أَنْ يُصَلِّيَ فَلْيَذْبَحْ مَكَانَهَا وَمَنْ لَمْ يَذْبَحْ فَلْيَذْبَحْ بِاسْمِ اللهِ»