Ibn Mas'ud (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) said, "When three of you are together, two of you must not converse privately ignoring the third till the number increases, lest the third should be grieved."[Al-Bukhari and Muslim].
App reference: Book 18, Hadith 89
Arabic/English book reference: Book 18, Hadith 1599