Muhammad says: Whoever recites "Rasul Allah" (the Messenger of Allah) in the accusative case (nasb), it is based on the meaning: "But he was the Messenger of Allah."
قَالَ مُحَمَّدٌ: مَنْ قَرَأَ (رَسُولَ الله) بِالنَّصْبِ فَعَلَى مَعْنَى: وَلَكِنْ كَانَ رَسُول الله.