How many stages were involved in the فرض (imposition) of prayer?

The prayer was enjoined on the night of the Isra' (Night Journey) five or six years before the Hijrah (migration), and it was said: about a year after the Prophetic mission began. A group of scholars held that there was no obligatory prayer before the Isra' except the night prayer, without specifying its form, while Al-Harbi maintained that prayer was obligatory, consisting of two rak'ahs in the morning and two rak'ahs in the evening. Imam Al-Shafi'i narrated from some people of knowledge that the night prayer was obligatory then abrogated by the verse: "So recite what is easy [for you] of it" (73:20), making it obligatory to pray part of the night. This was then abrogated by the five daily prayers.