Whoever says "I am free of Islam" is as he said

It is narrated from Sayyiduna Buraydah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever says, 'I am free of Islam', then if he is lying, he is as he said; and if he is truthful (in his claim), he will never return to Islam safe and sound."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

Sayyiduna Ali (may Allah be pleased with him) narrates in the square of Kufah: When the Messenger of Allah (peace and blessings be upon him) conquered Makkah, some people of Quraysh came to him and said: Some of our slaves and servants have joined you; their purpose is not the religion but only to flee from our livestock and our fields. The Messenger of Allah (peace and blessings be upon him) said: "O people of Quraysh! By Allah, you will surely establish the prayer and pay the zakah, or else I will send against you a man who will strike your necks for the sake of the religion." Then he said: "It will be me, or the one who is mending the sandal." Sayyiduna Ali says: At that time I was mending the sandal of the Messenger of Allah (peace and blessings be upon him). Then Sayyiduna Ali (may Allah be pleased with him) said: I heard the Prophet (peace and blessings be upon him) say: "Whoever lies about me will enter the Fire."
This hadith is sahih (authentic) on the condition of Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Zayd (may Allah be pleased with him) that once the Messenger of Allah (peace and blessings be upon him) was sitting with his Companions talking with them; then he (peace and blessings be upon him) got up and went (into his house). Zayd (who was the youngest of them) sat in his (peace and blessings be upon him) place and began narrating to the Companions the sayings of the Prophet (peace and blessings be upon him). Meanwhile, a gift of meat came for the Messenger of Allah (peace and blessings be upon him). The people said to Zayd: O Abu Sa'id! Go to the Prophet (peace and blessings be upon him), convey our greetings of peace to him and say: Your Companions say that, if you see fit, you might send us some of this meat. (When Zayd went,) he (peace and blessings be upon him) said: "Go back to them, for they have already eaten meat after you left." Zayd came back and told them: I have conveyed your message, but he (peace and blessings be upon him) said that they have already eaten meat. The people said: We have not eaten any meat; this seems to be something significant. Let us go and ask the Messenger of Allah (peace and blessings be upon him) ourselves. They came and said: O Messenger of Allah! We sent word to you about the meat, and Zayd claims that we have already eaten meat, yet by Allah, we have not eaten any meat. The Messenger of Allah (peace and blessings be upon him) said: "It is as though I can see the green (trace) of Zayd's flesh in your teeth" (meaning: you have backbitten him by talking about him behind his back). They said: O Messenger of Allah! Seek forgiveness for us. So he (peace and blessings be upon him) prayed for forgiveness for them.
This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it.

Sayyiduna Suwayd bin Hanzalah (may Allah be pleased with him) narrates: We set out to go to the Messenger of Allah (peace and blessings be upon him), and Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) was with us. On the way, an enemy of his seized him. The people hesitated to swear an oath (to free him), but I swore that he was my brother, so the enemy let him go. When we came to the Messenger of Allah (peace and blessings be upon him), I told him (peace and blessings be upon him) that the others had hesitated while I had sworn that he was my brother. He (peace and blessings be upon him) said: "You spoke the truth; a Muslim is the brother of a Muslim (so this oath of yours was not false)."
This hadith has a sahih (authentic) chain, but the two Shaykhs (Bukhari and Muslim) did not record it.