"Peace" shall be the greeting from the merciful Lord.
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Tafseer
'Abdullāh Ibn 'Abbās / Muḥammad al-Fīrūzabādī
تفسير : (the word from a merciful lord (for them) is: peace) they are greeted with the greeting of peace!
Jalāl al-Dīn al-Maḥallī
تفسير : “peace!” (salāmun is a subject) — the word (qawlan is its predicate), that is, [peace] by way of a word, from a lord [who is] merciful, to them, in other words, he says to them, ‘peace be on you!’