Respected Mufthi Sahib, Assalmu Alaikum!
Is it mentioned explicitly or indirectly that whosoever who do not believe in prophet Muhammad Sallallahu Alaihi wa Sallam is a Kafir?
Yes. From the common meaning of the following Ayaah of Qur’an, it is clearly explained the status of Kufr of a person who do not believe in Nabi Akram Sallallahu Alaihi wa Sallam.
Sura e Nisa, Ayath 150.151
Sura e Thowba, Ayath 84
Sura e Thaghabun, Ayath 6
Allah knows best.