I was present at the festival on the day of sacrifice along with God’s Messenger. He had done no more than finish his prayer and give the salutation when he saw the flesh of sacrificial animals which had been sacrificed before he finished his prayer. So he said, “Anyone who has sacrificed before he prayed (or, we prayed) must sacrifice another in place of it." In a version he said: The Prophet prayed on the day of sacrifice, then delivered a sermon, then sacrificed, and he said, “Anyone who has sacrificed before he prayed (or, we prayed) must sacrifice another in place of it, and if anyone has not sacrificed he should do so in God’s name."
Abu Huraira reported God's Messenger as saying, "There are no days on which God likes better to be worshipped than the ten days of Dhul Hijja. Fasting observed on each of these days is equivalent to a year’s fasting, and prayer during each of these nights is equivalent to prayer during Lailat al-qadr.’*
* A night towards the end of Ramadan. See Book 7, Chap. 9.
Tirmidhi and Ibn Majah transmitted it, but Tirmidhi said its isnad is weak.
