Jami` at-Tirmidhi Book 10, Hadith 58
"The Prophet prayed and in the funeral prayer he for An-Najashi, said four Takbir."
"I buried my son Sinan and Abu Talhah Al-Khawlani was sitting on the rim of the grave. When I wanted to leave he took me by my hand and said: 'Shall I not inform you of some good new O Abu Sinan!' I said: 'Of course.' He said: 'Ad-Dahhak bin Abdur-Rahman bin Arzab narrated to me, from Abu Musa Al-Ash'ari: "The Messenger of Allah said: 'When a child of the slave (of Allah) died, Allah says to the angels: "Have you taken the fruits of his work." They reply: "Yes." So He says: "What did My slave say?" They reply: "He praised you and mentioned that to You is the return." So Allah says: "Build a house in Paradise for My slave, and name it 'the house of praise.'"
"Zaid bin Arqam would say four Takbir for our funerals. (Once) he said five Takbir for a funeral so we asked him about that and he said: 'The Messenger of Allah would say those Takbir.'"