Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said regarding "Hajj is (in) well-known months" [Surah al-Baqarah: 197]: They are Shawwal, Dhu al-Qa'dah and ten days of Dhu al-Hijjah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8713