Majma al-Zawaid — Hadith 13137

Sayyiduna Ya'la bin Siyabah (may Allah be pleased with him) narrates that he knew this about the Prophet (peace and blessings be upon him): he came to a grave whose occupant was being punished, and he said: "This man used to eat the flesh of people." Then he called for a fresh branch, placed it on his grave and said: "Perhaps, as long as this branch remains fresh, (the punishment of) this man will be lightened."
This narration was recorded by Imam al-Tabarani in al-Mu'jam al-Awsat, and by Imam Ahmad within a long hadith which will come later in the chapter on the signs of prophethood. Its chain contains a narrator, Asim bin Bahdalah, who is thiqah (trustworthy) but has some weakness. The remaining narrators of this narration are thiqat (trustworthy).