It was narrated from ‘Uthman bin ‘Affan that the Prophet (ﷺ) offered the funeral prayer for ‘Uthman bin Maz’un, and he said four Takbir over him.
Sunan Ibn Majah Book 6, Hadith 70
← Previous Hadith “The Messenger of Allah (ﷺ), Abu Bakr and ‘Umar did not give us so much leeway in anything as they did with regard to the prayer for the deceased,” meaning that there was nothing affixed.

Sunan Ibn Majah Book 6, Hadith 69 Next Hadith → “I prayed with ‘Abdullah bin Abi Awfa Al-Aslami, the Companion of the Messenger of Allah (ﷺ), offering the funeral prayer for a daughter of his. He said Takbir over her four times, and he paused for a while after the fourth. I heard the people saying Subhan- Allah to him throughout the rows. Then he said the Salam and said: ‘Did you think that I was going to say a fifth Takbir?’ They said: ‘We were afraid of that.’ He said: ‘I was not going to do that, but the Messenger of Allah (ﷺ) used to say four Takbir, then pause for a while, and he would say whatever Allah willed he should say, then he would say the Salam.’”

Sunan Ibn Majah Book 6, Hadith 71