Musnad Ahmad 32:1775

Surayj narrated to us, Fulayh narrated to us, from Hilal ibn Ali, from Anas ibn Malik, who said: Some of the companions of the Prophet, may Allah bless him and grant him peace, whom I do not accuse, informed me that he said: While the Messenger of Allah, may Allah bless him and grant him peace, and Bilal were walking in al-Baqi’, the Messenger of Allah, may Allah bless him and grant him peace, said: "O Bilal, do you hear what I hear?" He said: "No, by Allah, O Messenger of Allah, I do not hear it." He said: "Do you not hear the people of these graves being punished?" meaning the graves of the Jahiliyyah.