Musnad Ahmad 541:15

Abu al-Yaman narrated to us, he said: Isma'il ibn 'Ayyash narrated to us, from 'Abd al-'Aziz ibn 'Ubayd Allah, from 'Abd Allah ibn al-Harith, he said: I heard 'Amr ibn al-'As saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "While I was in my sleep, the angels came to me and carried the pillar of the Book from under my pillow, and they headed with it towards al-Sham. Behold, indeed, faith, when the tribulations occur, is in al-Sham."