Is unbuttoning any of the garment's buttons a Sunnah for which the doer is rewarded, and did some of the Salaf do it as an act of worship and in adherence to the Sunnah?

The question refers to the hadith (definition) narrated by Abu Dawud from Qurrah ibn Iyas concerning the Prophet's shirt, which was "open-buttoned" (meaning its buttons were open). This hadith does not mean that leaving buttons open is an act of worship for which a Muslim is rewarded; rather, it is a permissible act. Not everything the Prophet (peace be upon him) did is considered an act of worship to be emulated.

One must differentiate between what the Prophet (peace be upon him) did as an act of worship and drawing closer to Allah, and what he did due to his human nature or the customs of people. The first type is what is legislated to be emulated, while the second is merely permissible.

Some scholars have used the hadith of Qurrah ibn Iyas as evidence for the permissibility of leaving buttons open, not for it being a Sunnah (definition). This is how Ibn Hibban and Al-Nawawi categorized it in their headings.

Ibn Taymiyyah mentioned that following the Prophet (peace be upon him) means conforming to him in both form and intention. He also stated that some Companions, like Ibn Umar, were meticulous in following everything the Prophet (peace be upon him) did, while the majority of the Companions believed that these actions were not Sunnah, but rather ordinary matters.

The Permanent Committee for Scholarly Research and Ifta issued a fatwa (definition) prohibiting a book whose author claimed that leaving buttons open was a Sunnah, due to its conflation of customary Sunnahs with ritual Sunnahs.

Sheikh Al-Albani affirmed that this action falls under the category of customary Sunnahs, not ritual worship, and that leaving buttons open might be for comfort in hot weather, not as an intended act of worship.

Sheikh Ibn Uthaymeen added that this action is not a regular Sunnah of the Prophet (peace be upon him), but rather a temporary matter due to a reason like intense heat. Leaving buttons open without a reason is considered frivolous, and it is a breach of chivalry if the chest is exposed.