Ubaydullah bin Adi bin al-Khiyar (may Allah have mercy on him) said that a man was speaking privately with the Prophet (peace and blessings be upon him). It was not known what he was saying, until the Prophet (peace and blessings be upon him) himself made it public that he was asking about killing a man from among the hypocrites. He (peace and blessings be upon him) said: "Does he not bear witness that there is no god but Allah?" He said: Of course! But his testimony counts for nothing. He (peace and blessings be upon him) asked: "Does he not pray?" He said: Of course, but it is of no benefit to him. He (peace and blessings be upon him) said: "Those are the ones I have been forbidden to kill."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16825