Musannaf Ibn Abi Shayba 6:990

Khalid ibn Makhlad narrated to us, from Abd al-Malik ibn al-Hasan al-Jari, from Abd Allah ibn Sa'd al-Jari, who said: Abu Hurayrah said to me: "O Abd Allah, when you pass by graves that you used to know, say: 'Peace be upon you, O inhabitants of the graves.' And when you pass by graves that you do not know, say: 'Peace be upon the Muslims.'"