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Narrated Uthman ibn Affan: Whenever the Prophet (ﷺ) became free from burying the dead, he used to stay at him (i.e. his grave) and say: Seek forgiveness for your brother, and beg steadfastness for him, for he will be questioned now. Abu Dawud said: The full name of the narrator Buhair is Buhair b. Raisan.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى الرَّازِيُّ، حَدَّثَنَا هِشَامٌ، عَنْ عَبْدِ اللَّهِ بْنِ بَحِيرٍ، عَنْ هَانِئٍ، مَوْلَى عُثْمَانَ عَنْ عُثْمَانَ بْنِ عَفَّانَ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم إِذَا فَرَغَ مِنْ دَفْنِ الْمَيِّتِ وَقَفَ عَلَيْهِ فَقَالَ " اسْتَغْفِرُوا لأَخِيكُمْ وَسَلُوا لَهُ التَّثْبِيتَ فَإِنَّهُ الآنَ يُسْأَلُ " . قَالَ أَبُو دَاوُدَ بَحِيرُ بْنُ رَيْسَانَ .
sunnah.com