Abdullah bin Mas'ud (may Allah be pleased with him) said in explaining "Hajj is (in) well-known months" [Surah al-Baqarah: 197]: They are Shawwal, Dhu al-Qa'dah and the first ten days of Dhu al-Hijjah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8712