What is the authenticity of the obligation to slaughter an animal for the deceased, called "the slaughter of the grave or the pit," between Fajr prayer and Eid prayer for the first time?

Worship is based on divine injunctions. This act was not part of the guidance of the Prophet (peace be upon him) nor his Companions, and it has no legal basis. Rather, it is an innovated Bid‘ah (innovation) whose performance is impermissible, due to the Prophet's (peace be upon him) saying: "Whoever innovates into this matter of ours something that does not belong to it, will have it rejected."