Sahih Muslim · Hadith 5823
The Book on Salutations and Greetings - كتاب السلام
حدثنا يحيى بن يحيى، قال قرأت على مالك عن نافع، عن ابن عمر، أن رسول الله صلى الله عليه وسلم قال " إذا كان ثلاثة فلا يتناجى اثنان دون واحد " .
Ibn Umar reported Allah's Messenger (may peace be upon him) as saying: When there are three (persons), two should not converse secretly between themselves to the exclusion of the (third) one.
Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).