I prayed behind Ibn Abbas at a funeral, and he recited the Opening of the Book and another surah aloud, loud enough for us to hear. When he finished, I took him by the hand and asked him about it.He replied, "It is the established practice and the truth."
أخبرنا الهيثم بن أيوب قال حدثنا إبراهيم وهو ابن سعد قال حدثنا أبي
صليت خلف ابن عباس على جنازة فقرأ بفاتحة الكتاب وسورة وجهر حتى أسمعنا فلما فرغ أخذت بيده فسألته فقال سنة وحق
Bukhari narrated it in the Book of Funerals, Chapter on Reciting Fatihah of the Book over the Funeral, hadith no. 1335, from the hadith of Saad bin Ibrahim through this same chain, and it is also in Kubra, hadith no. 2114.