Abu Abdullah, Imam Ahmad bin Hanbal (may Allah have mercy on him), reports from Mujahid (may Allah have mercy on him) concerning "The postponing (of a sacred month) is only an increase in disbelief" [Surah al-Tawbah: 37]: They performed Hajj in Dhu al-Hijjah for two years, then they performed Hajj in Muharram for two years.
Then they performed Hajj in each month for two years, until the Hajj of Abu Bakr al-Siddiq (may Allah be pleased with him) fell in Dhu al-Qa'dah, in the last of the two years, one year before the Hajj of the Prophet (peace and blessings be upon him). Then the Prophet (peace and blessings be upon him) performed Hajj the following year in Dhu al-Hijjah, and that is when the Prophet (peace and blessings be upon him) said in his sermon: "Time has returned to its original state, as it was when Allah, the Lord of Might, created the heavens and the earth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9776