"Ibn Abbas performed Salat for a funeral and he recited Fatihatil-Kitab. So I asked him about it and he said: 'It is from the Sunnah' or, 'From the completeness of the Sunnah.'" (Using translation from Tirmidhī 1027)
إِنَّهُ مِنَ السُّنَّةِ أَوْ مِنْ تَمَامِ السُّنَّةِ