The Arabs used to designate one year as a month and two years as two months, so they would only correctly observe the Hajj during the actual days of Hajj once every twenty-five years. This is the nasi (postponement) that Allah mentioned in His Book.When Abu Bakr led the people in Hajj, that year happened to coincide with the correct time of Hajj, and Allah called it the Greatest Hajj. The Messenger of Allah ﷺ performed Hajj the following year, and the people had begun observing the new moons properly.The Messenger of Allah ﷺ said, "Time has come full circle to the way it was on the day Allah created the heavens and the earth."
كانت العرب يجعلون عاما شهرا، وعامين شهرين، فلا يصيبون الحج في أيام الحج إلا في خمس وعشرين سنة مرة، وهو النسيء الذي ذكره الله في كتابه. فلما حج أبو بكر بالناس وافق العام الحج، فسماه الله الحج الأكبر، وحج رسول الله ﷺ من العام المقبل، فاستقبل الناس الأهلة. فقال رسول الله ﷺ: إن الزمان قد استدار كهيئة يوم خلق الله السموات والأرض.
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Tahawi related it in Sharh Mushkil Athar (1457) from Jafar bin Muhammad bin Hasan Firyabi, who said, Salt bin Masud Jahdari told us, who said, Muhammad bin Abdul-Rahman Tufawi told us, Dawud bin Abu Hind told us, from Amr bin Shuayb, from his grandfather, and so he related it.Its isnad is hasan because of Amr bin Shuayb and Muhammad bin Abdul-Rahman Tufawi, for both of them make a hadith hasan when they are not contradicted.Some scholars of knowledge said, the Prophet ﷺ only delayed the Hajj so that it would coincide with the reckoning of the people who calculated the calendar, but when the time returned to its original state, the Prophet ﷺ performed the Hajj so that it would coincide with the Hajj of the people after him until the Day of Resurrection.