Prohibition of two holding Secret Counsel to the Exclusion of Conversing together a Third

Allah, the Exalted, says:

"Secret counsels (conspiracies) are only from Shaitan (Satan)." (58:10)

Ibn 'Umar (May Allah be pleased with them) said: the Messenger of Allah (PBUH) said, "In the presence of three people, two should not hold secret counsel, to the exclusion of the third."

[Al-Bukhari and Muslim]

Ibn Mas'ud (May Allah be pleased with him) said: The Messenger of Allah (PBUH) 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]

The Hadith tell us that holding private counsel has been prohibited for the reason that it hurts the feelings of the Muslim who is ignored; and to hurt the feelings of a Muslim is a great sin indeed.