كتاب الأضاحى
Read the English translation first, with the original Arabic directly underneath.
Jundab b. Sufyan reported:
I was with Allah's Messenger (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 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.
مَنْ كَانَ ذَبَحَ أُضْحِيَّتَهُ قَبْلَ أَنْ يُصَلِّيَ - أَوْ نُصَلِّيَ - فَلْيَذْبَحْ مَكَانَهَا أُخْرَى وَمَنْ كَانَ لَمْ يَذْبَحْ فَلْيَذْبَحْ بِاسْمِ اللَّهِ