Hadith 946
Abu `Amr (also called Abu `Abdullah and also Abu Laila) from `Uthman bin `Affan (May Allah be pleased with him) reported:
عن أبي عمرو -وقيل: أبو عبد الله، وقيل: أبو ليلى- عثمان بن عفان رضي الله عنه قال: كان النبي صلى الله عليه وسلمى الله عليه وسلم الله عليه وسلم الله عليه وسلم إذا فرغ من دفن الميت وقف عليه، وقال: "استغفروا لأخيكم وسلوا له التثبيت، فإنه الآن يسأل" ((رواه أبو داود)).
After the burial of a dead man, the Prophet (ﷺ) would stand by the grave and say, "Ask Allah to forgive your brother and supplicate for him for steadfastness because he is being questioned (about his deeds) now." [Abu Dawud].
In plain words
After a burial the Prophet would stand at the grave and tell the people to ask Allah to forgive the deceased and to keep them firm, because they are now being questioned.
Reference : Riyad as-Salihin 946 In-book reference : Book 6, Hadith 53