Musannaf Ibn Abi Shayba 6:989

Ubayd Allah ibn Musa narrated to us, from Ibn Abi Dhi'b, from Qurrah, from Amir ibn Sa'd, from his father, that he used to return from his estate and pass by the graves of the martyrs. He would say, "Peace be upon you, and we are, by you, the followers." Then he would say to his companions, "Will you not greet the martyrs so that they may return the greeting to you?"