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.  

كَانَ النَّبِيُّ ﷺ إِذَا فَرَغَ مِنْ دَفْنِ الْمَيِّتِ وَقَفَ عَلَيْهِ فَقَالَ اسْتَغْفِرُوا لأَخِيكُمْ وَسَلُوا لَهُ التَّثْبِيتَ فَإِنَّهُ الآنَ يُسْأَلُ  

[Machine] I heard Uthman ibn Affan say that the Messenger of Allah ﷺ passed by a funeral at a grave, and the person accompanying the deceased was being buried. The Messenger of Allah ﷺ said, "Seek forgiveness for your brother and ask Allah to grant him steadfastness, for he is now being questioned."  

سَمِعْتُ عُثْمَانَ بْنَ عَفَّانَ يَقُولُ مَرَّ رَسُولُ اللَّهِ ﷺ بِجِنَازَةٍ عِنْدَ قَبْرٍ وَصَاحِبُهُ يُدْفَنُ فَقَالَ رَسُولُ اللَّهِ ﷺ «اسْتَغْفِرُوا لِأَخِيكُمْ وَسَلُوا اللَّهَ لَهُ التَّثْبِيتَ فَإِنَّهُ الْآنَ يُسْأَلُ»  

"اسْتْغفِرُوا لأخيكُمْ وسَلُوا له التَّثبيتَ؛ فإِنَّهُ الآن يُسْألُ".