وَالنِّيَّةُ ... وَتَعْيِينُ الْفَرْضِ وَتَعْيِينُ الْوَاجِبِ، وَلَا يُشْتَرَطُ التَّعْيِينُ فِي النَّفْلِ
The decisive intention is in the heart. A Fard prayer and a specific Wajib prayer such as Witr must be identified in intention; a general Nafl does not require the same specific designation.
Verbalizing the intention is not required. The commentary records disagreement among later scholars: some disliked verbalization, while others allowed quiet words as an aid against distraction. It should not be taught as a Prophetic Sunnah.