Hadith: A man asked permission to see the Prophet (may Allah's peace and blessings be upon him). Thereupon, he said: Let him in. What an evil brother of his people he is!

عن عائشة رضي الله عنها أن رجلًا استأذن على النبي صلى الله عليه وسلم فقال: «ائْذَنُوا له، بئس أخو العَشِيرَةِ؟».
[صحيح] - [متفق عليه]

A man asked permission to see the Prophet (may Allah's peace and blessings be upon him). Thereupon, he said: "Let him in. What an evil brother of his people he is!"
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

A man asked permission to see the Prophet (may Allah's peace and blessings be upon him). So he said: "Let him in. What an evil brother -or son- of his people he is!" When the man sat with him, the Prophet (may Allah's peace and blessings be upon him) was cheerful and at ease with him. When the man left, ‘Ā’ishah said to him: "O Messenger of Allah, when you saw the man you said such and such about him, but then you met him with a cheerful face and you were at ease with him!" The Messenger of Allah (may Allah's peace and blessings be upon him) said: "O ‘Ā’ishah, when have you known me to be rude? The worst person in the sight of Allah on the Day of Resurrection is the one whom people avoid for their evil." That man was evil, and that was the reason why the Prophet (may Allah's peace and blessings be upon him) said about him in his absence what he deserves in order to warn people of his evil so that they would not be deceived by him. So if you see a person who is wicked and corrupt but has enchanted people with his eloquence, it is your duty to expose his corrupt character to people so that they would not be deceived by him. How many a man with an eloquent tongue and impressive style whom when you see you admire his figure and when he speaks you listen to his speech, but there is no good in him, so it is a duty to make this known to people. As to being nice to the man, the Prophet (may Allah's peace and blessings be upon him) did so in order to appease him. Scholars stated that appeasement is required in dealing with others, unlike adulation and compromise. Compromise that leads to forfeiting a religious obligation or committing a prohibition or overlooking it is absolutely impermissible; Allah, the Almighty, said: {They wish that you should compromise (in religion out of courtesy) with them, so they (too) would compromise with you} [Sūrat al-Qalam: 9]. As to appeasement and treating people in a way that realizes benefit and does not lead to the slightest harm, it is legitimate.