Sayyiduna Hudhayfah (may Allah be pleased with him) said: We attended a funeral with the Prophet (peace and blessings be upon him). When we reached the grave, the Prophet (peace and blessings be upon him) sat on its edge and began to look into it. Then he said: "In it the believer is squeezed so hard that his hama'il (neck and chest) are dislodged, and for the disbeliever it is filled with fire." The narrator then mentioned the whole hadith; it will come in full later in the chapter on Zuhd (asceticism), if Allah wills.
Ahmad recorded it. Its chain contains Muhammad bin Jabir, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجنائز / 4253