Is the reading of Yasin khatam in Masjid after someone dies a sunnah or a custom?

Almighty Allah Ta%E3la states in the Noble Qur%E3n, 'O` you who believe!! Obey Allah and His Rasul'.%0D%0A%0D%0AThe life of the Prophet (Sallallaahu %C1layhi Wasallam) is indeed the best example for us to follow. During the time of the Prophet (Sallallaahu %C1layhi Wasallam) many of his family members, his wives, daughters, uncles and his illustrious companions left the world whilst engaged in the most noble work and effort of uplifting the religion of Islam. Despite the Prophet's deep%0D%0Alove for them, he did not (on any single occasion) commemorate the innovative custom of 7 days, 40 days, 100 days, etc.%0D%0A%0D%0AThe Hadith is clear, in that, the immediate family should mourn for three days. There is no basis in the Shari%E1h to commemorate any Muslim's death at%0D%0Aintervals of 7 days, etc.%0D%0A%0D%0AIt is not exactly clear as to the source of celebrating forty days after death. We, as Muslims, follow the Noble Qur%E3n and the life model of our%0D%0Abeloved Rasul (Sallallaahu %C1layhi Wasallam).%0D%0A%0D%0Aand Allah Ta'ala Knows Best%0D%0A%0D%0AMufti Ebrahim Desai%0D%0A