Muhammad said: "Wa'dan" (a promise) is an infinitive (masdar); the meaning is: He promised the resurrection with a promise.
قَالَ محمدٌ: (وَعدا) مَصْدَرٌ؛ وَالْمَعْنَى: وَعَدَ بِالْبَعْثِ وَعْدًا.