In one narration the wording is: I said: O Messenger of Allah (peace and blessings be upon him)! What is your view about it? He (peace and blessings be upon him) said: "It is an ember of fire that you have hung between your shoulders."
There is disagreement over this hadith concerning Ubadah bin Nusay, and the hadith of Sayyiduna Ibn Abbas and Sayyiduna Abu Sa'id (may Allah be pleased with them) has a more authentic chain than it. It has also been narrated through another, broken (munqati') chain from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11683