Language:
كتاب العيدين
13
The Two Festivals (Eids)
(23)
Chapter: If the Imam is asked about something while he is delivering the Khutba
(23)
باب كَلاَمِ الإِمَامِ وَالنَّاسِ فِي خُطْبَةِ الْعِيدِ، وَإِذَا سُئِلَ الإِمَامُ عَنْ شَيْءٍ وَهْوَ يَخْطُبُ
Sahih al-Bukhari 985

Narrated Jundab:

On the day of Nahr the Prophet (ﷺ) offered the prayer and delivered the Khutba and then slaughtered the sacrifice and said, "Anybody who slaughtered (his sacrifice) before the prayer should slaughter another animal in lieu of it, and the one who has not yet slaughtered should slaughter the sacrifice mentioning Allah's name on it."

حَدَّثَنَا مُسْلِمٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنِ الأَسْوَدِ، عَنْ جُنْدَبٍ، قَالَ صَلَّى النَّبِيُّ صلى الله عليه وسلم يَوْمَ النَّحْرِ ثُمَّ خَطَبَ، ثُمَّ ذَبَحَ فَقَالَ ‏ "‏ مَنْ ذَبَحَ قَبْلَ أَنْ يُصَلِّيَ فَلْيَذْبَحْ أُخْرَى مَكَانَهَا، وَمَنْ لَمْ يَذْبَحْ فَلْيَذْبَحْ بِاسْمِ اللَّهِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 985
In-book reference : Book 13, Hadith 34
USC-MSA web (English) reference : Vol. 2, Book 15, Hadith 101
  (deprecated numbering scheme)