The difference between voluntary acts (nawafil), recommended acts (mustahabb), and etiquette (adab) is very difficult in terminology, and in fact, there is no difference among them. They can also be called voluntary (tatawwu') and recommended (mandub). Al-Bukhari stated this in "Kashf al-Asrar," 2/302-303, where he said: "As for the definition of voluntary acts: which is called recommended, mustahabb, and voluntary: it is said that it is what is better to do than to leave. It is also said that it is what the obligated person is praised for doing and not blamed for leaving. It is also said that it is what is required to be done legally, without any blame for leaving it at all." Ibn Abidin mentioned in "Manhat al-Khaliq," 2/278, following Ibn Najim in "Al-Bahr al-Ra'iq," 1/29: "Know that the scholars agree that there is no difference between mustahabb and mandub, and that what the Prophet (peace be upon him) consistently did while leaving what he did not do without excuse is a Sunnah. What he did not consistently do is mandub and mustahabb, even if he did not do it after he encouraged it. This is also mentioned in "Al-Tahrir." He said in "Rad al-Muhtar," 1/477, after defining etiquette: "He defined it in the beginning of 'Al-Hilya' with multiple definitions, and he said: the apparent is that it is equal to the mandub." Imam al-Huda Abu al-Layth al-Samarqandi defined voluntary acts at the end of "Al-Muqaddimah" as it is known in the explanations of "Al-Hidayah," where he said: "As for voluntary acts: it is what the Prophet (peace be upon him) did at one time and left at another time, and he mentioned its virtue to his nation." Thus, it is known that there is no difference among them, except that it is a matter of distributing the names to their synonymous terms legally; to enhance the speech, and to indicate that the name should not be devoid of its named entity. It is called voluntary (nawfal) in the sense that it is additional to the obligatory and required acts, and it increases the reward. It is called mustahabb in the sense that the legislator loves it and prefers it. It is called mandub in the sense that it is between its reward and virtue, from the recommendation of the deceased, which is: enumerating his merits. It is called voluntary (tatawwu') in the sense that the doer does it voluntarily, without being commanded to do it necessarily. For further reference, see: "Rad al-Muhtar," 1/123, "Al-'Inayah," 1/215, "Bada'i al-Sana'i," 1/24, "Al-Labab," 1/13, "Al-Tawdi'h" explaining the introduction of Abu al-Layth, page 74, "Majma' al-Anhar" 1: 16, "Al-Durr al-Muntaka" 1: 16, and Allah knows best.