Amr bin Murrah said: I heard Sa'id bin al-Musayyib narrating from Sayyiduna Umar (may Allah be pleased with him) that he said: "All of these takbirs used to be said, four as well as five. Then we agreed by consensus on four for the funeral."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6946