Ubaidullah bin Adi bin Khiyar (may Allah have mercy on him) narrates the hadith of the Messenger of Allah (peace and blessings be upon him) that the Messenger of Allah (peace and blessings be upon him) was sitting among the people when a man came and whispered something, but it was not known what he whispered until the Messenger of Allah (peace and blessings be upon him) spoke loudly, and it became clear that he was seeking permission to kill a hypocrite. So when the Messenger of Allah (peace and blessings be upon him) raised his voice, he was saying: "Does he not recite the kalimah?" The man replied: "Of course, O Messenger of Allah (peace and blessings be upon him), but there is no value to his kalimah." So the Messenger of Allah (peace and blessings be upon him) said: "Does he not pray?" The man said: "Of course, but there is no value to his prayer." So the Messenger of Allah (peace and blessings be upon him) said: "It is these people whose killing Allah, the Exalted, has forbidden me."
Hadith Referenceالسنن المأثورة / باب ما يحل من هتك حرمة مسلم / 610
Hadith Takhrijمؤطا امام مالك، قصر الصلاة في السفر، باب جامع الصلاة، رقم: 84۔