Musannaf Ibn Abi Shayba 6:646

Waki’ narrated to us, from Sufyan, from ‘Amir ibn Shaqiq, from Abu Wa’il, who said: ‘Umar gathered the people and consulted them regarding the *takbir* (saying "Allahu Akbar") over the funeral prayer. Some of them said, "The Messenger of Allah (may Allah bless him and grant him peace) used to say it five times." Others said, "He used to say it seven times." And others said, "He used to say it four times." He said: So he united them upon four *takbirs*, like the longest of the prayers.