Grading: This hadith is munkar and its chain is da'if.
Technical note / Defect ('illah): The scholars differed about Hurayth bin al-Sa'ib, and he is closer to weakness. He erred in making this report the speech of the Prophet (peace and blessings be upon him), whereas it is the speech of the People of the Book.
Important point: Imam al-Daraqutni says in "al-'Ilal" (265): "Hurayth bin al-Sa'ib narrated it from al-Hasan, from Humran, from 'Uthman, from the Prophet (peace and blessings be upon him), and he erred in it; the correct version is: from al-Hasan, from Humran, from one of the People of the Book."

Reference / Source: Al-Hafiz Ibn Hajar, in the entry on Hurayth in "Tahdhib al-Tahdhib", quoted Imam Ahmad as rejecting this hadith. Then he said: Al-Athram mentioned its defect from Ahmad: Ahmad was asked about Hurayth and said: This is a Basran shaykh who narrated a munkar hadith with the chain al-Hasan, Humran, 'Uthman. I said: Qatadah contradicts him. He said: Yes, Sa'id from Qatadah, from al-Hasan, from Humran, from a man of the People of the Book. Imam Ahmad said: Rawh narrated it to us, (saying) Sa'id narrated it to us, i.e. from Qatadah. Al-'Uqayli cited it in "al-Du'afa'" (369) and said: He is not corroborated in it. Ahmad recorded it in his "Musnad" (1/440) and al-Tirmidhi (2341) through 'Abd al-Samad bin 'Abd al-Warith, with this same chain. Al-Tirmidhi called it "a sahih hadith", even though it is the hadith of Hurayth bin al-Sa'ib!

Details of the narration: 'Abd Allah bin al-Mubarak narrated it from Hurayth, from al-Hasan al-Basri as mursal: The Messenger of Allah (peace and blessings be upon him) warned (the full hadith).

Reference / Source: Abu Sa'id bin al-A'rabi recorded it in "al-Zuhd wa Sifat al-Zahidin" (83) from Abu 'Amr al-Dabbi, from Mu'adh bin Asad, from Ibn al-Mubarak.
Corroborating chains and supporting reports: On this topic there is also a narration from Ibn 'Abbas (may Allah be pleased with them both) in al-Bazzar (3643, Kashf al-Astar) and in Abu Nu'aym's "al-Hilyah" (4/100), but its chain contains Layth bin Abi Sulaym, and he is da'if.

Note / Clarification: Regarding the words of the hadith "jilf min al-khubz", al-Tirmidhi says: I heard Abu Dawud Sulayman bin Salm al-Balkhi say that al-Nadr bin Shumayl said: "Jilf al-khubz" means bread with no condiment with it.

Note / Clarification: Ibn al-Athir says: "Al-jilf" is bread on its own with no condiment with it. One view is that it means coarse, dry bread. It is also narrated with a fathah on the lam ("jilaf"), the plural of "jilfah", which means a piece of bread.