All the Muhadditheen and Historians are unanimous on the fact that Rasulullah (sallallahu ‘alaihi wasallam) was bestowed with the mantle of prophethood on a Monday. However, there is a difference of opinion regarding the actual month in which he was blessed with nubuwwah.

Haafiz ibnu ‘Abdil Barr (rahimahullah) says that he was blessed with prophethood on the eighth of Rabee’-ul’Awwal. Hence, Rasulullah (sallallahu ‘alaihi wasallam) was exactly forty years old when he became the Nabi of Allah Ta‘ala.

According to Muhammed bin Ishaaq (rahimahullah), Rasulullah (sallallahu ‘alaihi wasallam) was blessed with prophethood on the 17th of Ramadhaan. According to this view, Rasulullah (sallallahu ‘alaihi wasallam) was forty years and six months at the time of receiving prophethood.

Haafiz ibnul Hajar (rahimahullah) has mentioned in Fat-hul Baari that the latter view is the preferred view and correct view, as it was in Ramadhaan that Rasulullah (sallallahu ‘alaihi wasallam) would remain in i’tikaaf in the cave of Hiraa’ (and it was during the i’tikaaf in Hiraa’ that Rasulullah (sallallahu ‘alaihi wasallam) was blessed with prophethood).

(Extracted from Seeratul Mustafa 1/145)