Language:
كتاب الجنائز
23  
Funerals (Al-Janaa'iz)
(66)
Chapter: To offer the (funeral) Salat on the grave after the burial of the deceased
(66)
باب الصَّلاَةِ عَلَى الْقَبْرِ بَعْدَ مَا يُدْفَنُ
Sahih al-Bukhari 1336

Narrated Sulaiman Ash-Shaibani:

I heard Ash-Shu`bi saying, "I was told by a man who had passed with the Prophet (p.b.u.h) by a grave that was separate from the other graves that he (the Prophet (ﷺ) ) led them in the prayer and they prayed behind him." I said, "O Abu `Amr! Who narrated that to you?" He replied, "Ibn `Abbas."

حَدَّثَنَا حَجَّاجُ بْنُ مِنْهَالٍ، حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنِي سُلَيْمَانُ الشَّيْبَانِيُّ، قَالَ سَمِعْتُ الشَّعْبِيَّ، قَالَ أَخْبَرَنِي مَنْ، مَرَّ مَعَ النَّبِيِّ صلى الله عليه وسلم عَلَى قَبْرٍ مَنْبُوذٍ فَأَمَّهُمْ وَصَلَّوْا خَلْفَهُ‏.‏ قُلْتُ مَنْ حَدَّثَكَ هَذَا يَا أَبَا عَمْرٍو قَالَ ابْنُ عَبَّاسٍ ـ رضى الله عنهما ـ‏.‏
Reference : Sahih al-Bukhari 1336
In-book reference : Book 23, Hadith 91
USC-MSA web (English) reference : Vol. 2, Book 23, Hadith 420
  (deprecated numbering scheme)