It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Two people should not converse privately to the exclusion of the third, for that hurts the believer, and Allah Almighty dislikes the hurting of the believer."
This hadith is not narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) except with this chain, and Abdullah bin al-Mubarak (may Allah have mercy on him) is alone in narrating it.
Hadith Referenceالمعجم الأوسط / باب الألف / 1986
Hadith Takhrij«أخرجه الضياء المقدسي فى «الأحاديث المختارة» برقم: 269، وأبو يعلى فى «مسنده» برقم: 2444، وابن حجر فى «المطالب العالية» برقم: 2690، والطبراني فى «الأوسط» برقم: 4988»