Usage in the Quran
Sukaina is the Spirit of Tranquility, or Peace of Reassurance it's also a shortened form of the original word "Sakinah" which is mentioned in the Quran as having descended upon the Islamic prophet Muhammad and the believers as they made an unarmed pilgrimage to Mecca, and were faced with an opposing military force of the Quraysh, with whom the Prophet struck the Treaty of Hudaybiyah. "He it is Who sent down the sakinah into the hearts of the believers that they might add faith unto their faith" (48:4)
Another Quranic association with the concord of dwellings in peace coincides with the attribution of the Shekhinah to matrimonial concord under the tent of Sarah: "And God gave you your houses as a quiescent place (sakanan)." (16:80)
Sakinah is further mentioned in the following verse: "While the Unbelievers got up in their hearts heat and cant - the heat and cant of ignorance,- Allah sent down His Sakinah - tranquility (sakīnatahu) to his Messenger and to the Believers, and made them stick close to the command of self-restraint; and well were they entitled to it and worthy of it. And Allah has full knowledge of all things." (48:26)
Sakinah and Shekhinah
Karen Armstrong notes: "The sakinah it will also be recalled, seems to be related to the Hebrew Shekhinah, the term for God’s presence in the world." Another Quranic verse portrays sakinah as reassurance: "Allah's Good Pleasure was on the Believers when they swore Fealty to thee under the Tree: He knew what was in their hearts, and He sent down Sakina - tranquillity (alssakeenata) to them; and He rewarded them with a speedy Victory;" (48:18)
The root of the word is sa-ka-nah which means "dwelled" or "remained in place". This further supports the association with the Shekhinah as "indwelling". The fact that the word is preceded by "al" (the) shows that it does not denote a name, but has an abstract meaning.
Sufi writings, in expounding the inner peace of Sufi contemplation, which dwells in a sanctuary or in the heart, confirm the association with both Sakinah and the Shekhinah. Sufi reference to sa-ka-na as meaning both stillness and habitation adds to the identity with Shekhinah's indwelling nature.