Abu Wa'il said that in the time of the Messenger of Allah (peace and blessings be upon him) seven, five and six takbirs used to be said, or he said: four. So Umar bin al-Khattab (may Allah be pleased with him) gathered the Companions, and each man reported what he had seen; then Umar bin al-Khattab (may Allah be pleased with him) united them on four takbirs, like the longest prayer.
Waki' narrated it from Sufyan with 'four' in place of 'six'. And in what Waki' narrated from Mis'ar, from Abd al-Malik bin Iyas al-Shaybani, from Ibrahim: the Companions of the Messenger of Allah (peace and blessings be upon him) gathered in the house of Abu Mas'ud al-Ansari (may Allah be pleased with him), and they agreed that the takbirs over the funeral are four.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6947