Musnad Ahmad 1:26

'Abd al-Razzaq narrated to us, he said: Ibn Jurayj informed me, he said: My father informed me: That the companions of the Prophet, may God bless him and grant him peace, did not know where to bury the Prophet, may God bless him and grant him peace, until Abu Bakr, may God be pleased with him, said: I heard the Messenger of God, may God bless him and grant him peace, saying: "A prophet is not buried except where he dies." So they moved his bed, and they dug for him beneath his bed.