Oneness, Uniqueness of Allah (Tawheed)

Bukhari :: Book 9 :: Volume 93 :: Hadith 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."