Musnad Ahmad 1078:1

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, Ibn Shihab informed me, from 'Ata' ibn Yazid al-Laythi, from 'Ubayd Allah ibn 'Adi ibn al-Khiyar, that a man from the Ansar narrated to him that he came to the Messenger of Allah (may Allah bless him and grant him peace) while he was in a gathering, and he whispered to him, seeking his permission to kill a man from the hypocrites. The Messenger of Allah (may Allah bless him and grant him peace) spoke aloud and said: "Does he not testify that there is no god but Allah?" The Ansari said: "Yes, O Messenger of Allah, but his testimony is not valid." The Messenger of Allah (may Allah bless him and grant him peace) said: "Does he not testify that Muhammad is the Messenger of Allah?" He said: "Yes, O Messenger of Allah, but his testimony is not valid." He said: "Does he not pray?" He said: "Yes, O Messenger of Allah, but his prayer is not valid." The Messenger of Allah (may Allah bless him and grant him peace) then said: "Those are the ones whom Allah has forbidden me from [killing]."