Majma al-Zawaid — Hadith 10024

Sayyidah Aishah al-Siddiqah (may Allah be pleased with her) narrates: The Prophet (peace and blessings be upon him) ordered that the slain (from among the polytheists) be thrown into the pit, and they were thrown into the pit, except Umayyah bin Khalaf, for this could not be done with him: he had swollen inside his coat of mail and filled it, and when the people tried to move him his body fell apart, so they left him where he was and threw earth and stones over him. When they had thrown the slain into the pit, the Prophet (peace and blessings be upon him) stood by their bodies and said: "O people of the pit, have you found what your Lord promised you to be true? For I have found what my Lord promised me to be true." His Companions said to him: "O Messenger of Allah, are you speaking to dead people?" After this the narrator mentioned the narration as before.
Ahmad recorded it, and its narrators are thiqat (trustworthy).