Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "All the days of Tashriq (that is, the 11th, 12th and 13th of Dhul-Hijjah) are days of sacrifice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19243