Performing the janaazah prayer in the graveyard after burial
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Question: If the deceased has been buried in the grave, is it permissible to pray (the janaazah prayer) over him?

Response: If the deceased has been buried in the grave, it is permissible to pray over him. And the evidence for this is that the Prophet (sal-Allaahu 'alaye wa sallam) went out to the graveyard and prayed over the grave of the woman who used to sweep the masjid, and who had died and the Prophet (sal-Allaahu 'alaye wa sallam) did not know about this.

Shaykh Ibn 'Uthaymeen
Liqaa.aat al-Baab al-Maftooh - Volume 1, Page 25, Number 35