Sayyiduna Abdullah bin Umar (may Allah be pleased with them) narrates: The Prophet Muhammad (peace and blessings be upon him) said: "Two people should not converse secretly with each other excluding their (third) companion, because this will make him sad." The narrator named Abu Salih says: I asked Sayyiduna Abdullah bin Umar (may Allah be pleased with them): What if there are four people? He replied: Then there is no harm to you.
Hadith Referenceصحیح ابن حبان / كتاب البر والإحسان / 584
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الصحيحة» -أيضا-.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط البخاري، رجاله ثقات رجال الشيخين غير مسدد، فمن رجال البخاري.