Mishkat al-Masabih 5 Funerals كتاب الجنائز 129

وَعَنْ طَلْحَةَ بْنِ عَبْدِ اللَّهِ بْنِ عَوْفٍ قَالَ: صَلَّيْتُ خَلْفَ ابْنِ عَبَّاسٍ عَلَى جَنَازَةٍ فَقَرَأَ فَاتِحَةَ الْكِتَابِ فَقَالَ: لِتَعْلَمُوا أَنَّهَا سُنَّةٌ. رَوَاهُ البُخَارِيّ

Talha b. ‘Abdallah b. ‘Auf said he prayed behind Ibn ‘Abbas at a funeral and he recited Fatihat al-Kltab (Qur’an, sura i). He then said that it was to let them know it was a sunna . Bukhari transmitted it.