حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا زُهَيْرٌ، حَدَّثَنَا الأَسْوَدُ بْنُ قَيْسٍ، ح وَحَدَّثَنَاهُ يَحْيَى، بْنُ يَحْيَى أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنِ الأَسْوَدِ بْنِ قَيْسٍ، حَدَّثَنِي جُنْدَبُ بْنُ سُفْيَانَ، قَالَ شَهِدْتُ الأَضْحَى مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فَلَمْ يَعْدُ أَنْ صَلَّى وَفَرَغَ مِنْ صَلاَتِهِ سَلَّمَ فَإِذَا هُوَ يَرَى لَحْمَ أَضَاحِيَّ قَدْ ذُبِحَتْ قَبْلَ أَنْ يَفْرُغَ مِنْ صَلاَتِهِ فَقَالَ ‏"‏ مَنْ كَانَ ذَبَحَ أُضْحِيَّتَهُ قَبْلَ أَنْ يُصَلِّيَ - أَوْ نُصَلِّيَ - فَلْيَذْبَحْ مَكَانَهَا أُخْرَى وَمَنْ كَانَ لَمْ يَذْبَحْ فَلْيَذْبَحْ بِاسْمِ اللَّهِ ‏"‏ ‏.‏

Jundab b. Sufyan reported: I was with Allah's Messenger (PBUH) 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. Translation Not Available

Book reference: Sahih Muslim Book 35 Hadith 4818
Web reference: Sahih Muslim Book 22 Hadith 1960 a