Musannaf Ibn Abi Shayba 6:627

Waki’ narrated to us, from Mis’ar, from Sufyan and Shu’bah, from ‘Ali ibn al-Aqmar, from Abu ‘Atiyyah, who said: ‘Abdullah said: "The takbir over the funeral prayers is four takbirs, with the takbir of exiting [the prayer]."