Tafsir of Ash-Shu`ara' 26:213

"So do not invoke with Allah another deity, lest you be among the punished."

The Prophet—peace and blessings be upon him—is addressed herein, despite the impossibility of the prohibited act occurring from him, as a means of agitation and incitement toward increasing in sincerity. It is a metonymy for sincerity in monotheism, so that one sees nothing alongside Him—Exalted and Majestic is He. Within this lies kindness toward all those burdened with religious obligations, by clarifying that associating partners with God is so ugly and evil that it is forbidden even to one from whom such an act could never proceed; so how much more so for those other than him? The fa (So) is fasiha (eloquent), meaning: If you have come to know what has been mentioned, then do not invoke with Allah another deity.