Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 2089
It is narrated on the authority of 'Abd al-Rahman b. Abu Laila that Zaid used to recite four takbirs on our funerals and he recited five takbirs on one funeral. I asked him the reason (for this variation), to which he replied: The Messenger of Allah (may peace be upon him) recited thus.