All perfect praise be to Allah. Blessings and peace be upon Prophet Mohammad.

The funeral prayer(Janaazah) is a collective duty: If it is performed by some Muslims, then the rest are absolved form liability. This prayer is legislated after the burial of the deceased for one who hadn`t performed it on him whether it was performed after, or before the burial. Narrated Ibn Abbass (May Allah be pleased with them): “A person died and Allah's Apostle used to visit him. He died at night and (the people) buried him at night. In the morning they informed the Prophet (about his death). He said: "What prevented you from informing me?" They replied: "It was night and it was a dark night and so we disliked to trouble you." The Prophet went to his grave and offered the (funeral) prayer” {Bukhari&Muslim}.

However, there is a key condition in this regard: Whoever wishes to offer the funeral prayer should be discerning and have reached puberty upon the burial of the deceased since a minor, at the time of burial, isn`t liable as far as the Sharia obligations are concerned, so it isn`t permissible for him to perform it. It is stated in [Sharh Al-Minhajj LlMahalli Ma` Hashiat A`aliobi] : “It is imperative that the funeral prayer precedes the burial, but it is valid to offer it afterwards at the grave, whether the deceased was buried after, or before it."

It is more correct to perform the prayer at the grave in case the prayer performer was grown up, of sound mind, ritually pure, and Muslim; otherwise, the prayer is invalid as the case with the minor. This is the predominant opinion of the Shafite jurisits.

Until when is it permissible to offer the funeral prayer on the deceased who had been buried: It was said until three days, until a month, until there are remnants of the corpse, and it was said until The Day of Resurrection, and this is the predominant opinion. And Allah Knows Best.