In The Name of Allah, The Most Merciful, The Bestower of Mercy.

Khaalid Bin Hayyaan narrated from Jafar Bin Burqaan [may Allah have mercy upon them] who said, “Maymoon Bin Mihraan [may Allah have mercy upon him] said to me, ‘Oh, Jafar! Say what I detest (i.e. the bitter truth) in my face since a man will not have (fulfilled) advice to his brother until he says in his face what he detests”. (1)

Umar Ibn Abdul Azeez [may Allah have mercy upon him] said, “Whoever advises his brother to rectify affairs related to his religion and gives him insight into a worldly affair has perfected their relationship and fulfilled his obligation of (brotherhood) to him”. (2)

Imaam Al-Barbahaaree [may Allah have mercy upon him] said, “It is not permissible to hide sincere advice from any of the Muslims, whether pious or impious, in matters of the religion. Whoever hides that has acted deceitfully towards the Muslims. Whoever acts deceitfully toward the Muslims has done so towards the religion. Whoever acts deceitfully towards the religion has behaved treacherously towards Allah, His Messenger, and the Believers”.

Al-Allaamah Ahmad Bin Yahyah An-Najmi [may Allah have mercy upon him] said: “I say: An-Naseehah means ‘purity’. When milk is clear, it is said, [نصح اللبن]. This is the linguistic meaning of An-Naseehah. In the Sharee’ah, (it is) to inform a Muslim regarding what is obligated to him in relation to the (rights of Allah) and (the rights of Allah’s) creation without slyness and false courtesy. An-Naseehah is obligatory, even though its obligation varies, and its opposite is Al-Ghish [deception]”. (3)

[Ref 1: Siyar A’laam An-Nubulaa. 5/75]

[Ref 2: Taareekh At-Tabari 6/572]

[Ref 3: Irshaad As-Saariy Fee Sharhis Sunnati Lil-Barbahaaree. Pages 157-158. slightly paraphrased]