ما الدليل على أن صلاة الصبح هي الفرض وصلاة الفجر هي النافلة؟

Fajr prayer is the Subh prayer. They are two names for one prayer among the five obligatory prayers. Its time begins at the break of dawn and ends at sunrise. Some jurists refer to the Sunnah prayer (preceding the obligatory one) as Fajr, and the obligatory prayer as Subh.