I'm looking for a hadith which the khatheeb mentioned today. He mentioned that the hadith permits us to sacrifice for Dhul-Hijjah on Eid, or any of the three days following 'Eid. (For example, if Eid is on Friday, the hadith permits sacrificing up to, and including, Monday.)
It's probably this hadith narrated in Sunan Ahmad:
عَنْ جُبَيْرِ بْنِ مُطْعِمٍ عَنْ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ كُلُّ عَرَفَاتٍ مَوْقِفٌ وَارْفَعُوا عَنْ بَطْنِ عُرَنَةَ وَكُلُّ مُزْدَلِفَةَ مَوْقِفٌ وَارْفَعُوا عَنْ مُحَسِّرٍ وَكُلُّ فِجَاجِ مِنًى مَنْحَرٌ وَكُلُّ أَيَّامِ التَّشْرِيقِ ذَبْحٌ
Narrated by Jubair ibn Mut'im from the Prophet: "All Arafat is a standing-place,, except the middle of Urana, and the whole of Muzdalifa is a standing-place, except for the middle of Muhassir, and all pathways of Mina is a sacrifice place, and all the days of Tashriq are days of sacrifice.
The days of Tashriq are of course the three days following Yaum Annahr (day of sacrifice). This hadith is a matter of contention, as many scholars classify the last part "all the days of Tashriq are days of sacrifice" as da'eef. However, Imam Ashafi'ee and others have accepted it.