Hadith: The liar is not the one who seeks reconciliation between people, conveying good, or saying good things

عن أم كلثوم بنت عقبة بن أبي مُعَيط رضي الله عنها قالت: سمعت رسول الله صلى الله عليه وسلم يقول: «ليس الكذاب الذي يُصلح بين الناس فَيَنْمِي خيرًا، أو يقول خيرًا» وفي رواية مسلم زيادة، قالت: ولم أسمعه يُرَخِّصُ في شيء مما يقوله الناس إلا في ثلاث، تعني: الحرب، والإصلاح بين الناس، وحديث الرجل امرأته، وحديث المرأة زوجها.
[صحيح] - [متفق عليه]

Umm Kulthūm bint ‘Uqbah ibn Abi Mu‘ayt (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: “The liar is not the one who seeks reconciliation between people, conveying good, or saying good things.” There is an addition in the narration of Muslim, saying: that I did not hear him giving permission of lying in anything except in three (things): the war, reconciliation between people, and what a man says to his wife and what a woman says to her husband.
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

The basic rule states that lying is unlawful, for the Prophet (may Allah's peace and blessings be upon him) said: “Beware of telling lies, for lying leads to wrongdoing and wrongdoing leads to hellfire. A man will keep telling lies and strive to tell lies until he is recorded with Allah as a liar” [Muslim]. However, there are three exceptions where lying is permissible: Lying for the sake of reconciliation between people; lying during war; and lying in what a man says to his wife. The Sunnah has made lying in these three cases permissible, for what it contains of benefits without resulting in any harm, as following: 1. Lying for the sake of reconciliation between two people or two tribes disputing with each other. Here, one conveys good words and narrates to one of the disputing parties that the other praised him and spoke well of him and mentioned him with good attributes. He in fact did not hear so, however, his aim is to bring them together and to remove the aversion and estrangement between them. This is permissible and there is no harm in it as long as one's aim is reconciliation and removing enmity and hatred from the hearts. 2. Lying during war: it is to display strength and say things to empower his companions, and to increase their morale, in order to defeat the enemies. It also includes saying that the Muslim army is big in number and has received strong back up, or saying: Look behind you! So-and-so has come from behind you to strike you! This is also permissible, due to what it contains of enormous benefit for Islam and the Muslims. 3. Lying between spouses: i.e. husband says to his wife "You are the most beloved person to me, and I desire only you” and similar words that induce harmony and love between them; and she says the same. This is permissible as well, due to what it contains of mutual benefits. It is noteworthy that lying between the spouses is limited to what will bring or increase harmony and love between them. It is not permissible in everything. An-Nawawi (may Allah have mercy upon him) said: “Regarding man's lying to his wife and vice versa, it is intended to be a means of displaying love and devotion to one another and similar things. However deception, for the sake of evading duties or taking what is not his or her right, this is unlawful according to the scholars' consensus” [Sharh Muslim by An-Nawawi]. Al-Hafizh Ibn Hajar (may Allah have mercy upon him) said: “They (scholars) agreed that the permissible lying between a wife and a husband is in those things which do not entail the negligence of the duties of any of them or the acquisition of rights that they are not entitled to.”