"It has been decreed for him," meaning: it has been ordained for the devil, "that whoever turns to him [as an ally]," meaning: follows him, "he will misguide him."
Muhammad said: "(that whoever turns to him [as an ally])"—the particle "that" (anna) is in the nominative position, and "(he will misguide him)" is a conjunctive clause to it, and its position is likewise nominative. Its reality is that it is reiterated for the sake of emphasis; the meaning is: It has been decreed for him that whoever follows him, he shall misguide him.
Surat al-Hajj (Verse 5).
﴿كتب عَلَيْهِ﴾ أَيْ: قُضِيَ عَلَى الشَّيْطَانِ ﴿أَنَّهُ من تولاه﴾ اتبعهُ ﴿فَأَنَّهُ يضله﴾.
قَالَ مُحَمَّدٌ: (أَنَّهُ مَنْ تَوَلاهُ) (أَنَّهُ) فِي مَوْضِعِ رَفْعٍ، (فَأَنَّهُ يضله) عَطْفٌ عَلَيْهِ، وَمَوْضِعُهُ رَفْعٌ أَيْضًا، وَحَقِيقَتُهُ أَنَّهَا مِكَرَّرَةٌ عَلَى جِهَةِ الْتَوْكِيدِ؛ الْمَعْنَى: كُتُبِ عَلَيْهِ أَنَّهُ من تولاه أضلّهُ.
سُورَة الْحَج (آيَة ٥).