Sahih Bukhari
Oneness, Uniqueness of Allah (Tawheed)
Volume: 009 / Book Number: 093 / Hadith Number: 497
Narrated Jundab:
That he witnessed the Prophet on the Day of Nahr. The Prophet offered prayer and then delivered a sermon saying, "Whoever slaughtered his sacrifice before offering prayer, should slaughter another animal in place of the first; and whoever has not yet slaughtered any, should slaughter a sacrifice and mention Allah's Name while doing so."