Musnad Abu Ya'la al-Mawsili 110:117

Abu al-Rabi' al-Zahrani narrated to us, Ibn al-Mubarak narrated to us, from 'Abd al-Wahhab ibn al-Ward, from al-Hasan ibn Habib—or Kathir—from 'Ikrimah, from Ibn 'Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Two people must not whisper to each other excluding the third, for that causes him harm, and Allah dislikes the harming of the believer."