Sahih Muslim Book - 35, Hadith - 5065

وحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا أَبُو الأَحْوَصِ سَلاَّمُ بْنُ سُلَيْمٍ ، عَنِ الأَسْوَدِ بْنِ قَيْسٍ ، عَنْ جُنْدَبِ بْنِ سُفْيَانَ ، قَالَ : شَهِدْتُ الأَضْحَى مَعَ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، فَلَمَّا قَضَى صَلاَتَهُ بِالنَّاسِ نَظَرَ إِلَى غَنَمٍ قَدْ ذُبِحَتْ ، فَقَالَ: مَنْ ذَبَحَ قَبْلَ الصَّلاَةِ ، فَلْيَذْبَحْ شَاةً مَكَانَهَا ، وَمَنْ لَمْ يَكُنْ ذَبَحَ ، فَلْيَذْبَحْ عَلَى اسْمِ اللَّهِ.

It was narrated that Jundab bin Sufyan said: "I was present at ('Eid) Al-Adha with the Messenger of Allah (s.a.w), and when he had finished leading the people in prayer, he looked towards some sheep that had been slaughtered and said: 'Whoever slaughtered (his sacrifice) before the prayer, let him slaughter a sheep in its stead, and whoever has not yet slaughtered (his sacrifice), let him slaughter it in the Name of Allah."'