When was the funeral prayer legislated, and why?

Scholars differ regarding the exact time when the funeral prayer (Salat al-Janazah) was legislated, and there is no explicit evidence that pinpoints it. Some scholars have inferred that it was not legislated in Mecca, but rather in Medina in the first year of the Hijra. They base this on the fact that the Prophet, peace and blessings be upon him, prayed over Al-Bara' ibn Ma'rur in Medina after he died in Mecca, and that the funeral prayer was not legislated on the day Khadijah passed away in Mecca. Others have indicated the possibility that it was legislated in Mecca after Khadijah's death and before the Hijra.