فَقُلْتُ اسْتَغْفِرُوا رَبَّكُمْ إِنَّهُ كَانَ غَفَّارًا
I said: seek forgiveness from your Lord; He is most forgiving.
The derivational relation between “istaghfirū” and “ghaffārā” ties the opening and closing parts of the utterance through sound and meaning. The figure radd al-ʿajz ʿalā al-ṣadr requires not just a shared root but an apt positional relation.
If related forms occurred incidentally in unrelated middle positions, the label would require further justification. Both derivational link and placement matter.