Musnad Ahmad 27:1878

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ayyub, from Nafi', from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If you are three, then two must not whisper privately excluding the third, except with his permission, for that would sadden him."