Ata bin Abi Rabah (may Allah have mercy on him) said: Allah Almighty said, "The Hajj is (in) the well-known months" [Surah al-Baqarah: 197], only so that Hajj would not be undertaken in other than them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8722