Musnad Abu Ya'la al-Mawsili 130:32

Ja'far narrated to us, 'Abd al-Warith narrated to us, from 'Abd al-'Aziz ibn Suhayb, from Anas, who said: There was a Christian man who embraced Islam during the time of the Messenger of Allah (may Allah bless him and grant him peace), and he recited [the surahs of] al-Baqarah and Al 'Imran. He used to write for the Prophet of Allah (may Allah bless him and grant him peace). Then, he reverted to being a Christian and used to say, "Muhammad does not know anything except what I used to write for him." Allah caused him to die, so they buried him. The next morning, the earth had cast him out. They said, "This is the work of Muhammad and his companions; they were not pleased that he left their religion, so they dug up our companion." They dug for him and made the grave deeper. The next morning, the earth had cast him out. They said, "This is the work of Muhammad and his companions; they dug up our companion and threw him out." They dug for him and made it as deep in the earth as they could. The next morning, the earth had cast him out. So they knew that it was not from the people, but rather from Allah (Mighty and Majestic is He), and they left him.