Musnad Ahmad 27:1564

'Ali ibn 'Ayyash narrated to us, Shu'ayb ibn Abi Hamzah narrated to us, from Nafi', from 'Abdullah ibn 'Umar, from the Prophet (peace and blessings of Allah be upon him), who said: "If three are gathered, then let not two whisper privately to the exclusion of the third. And let not one of you make his brother stand up from his seat, then sit in it."