What is the authenticity of the حديث (Hadith) that mentions a man passing by the Prophet's Mosque when it was empty, and his saying: "Do you know that this place used to be full of people?"

It is authentically reported in Sahih Muslim from Abu Hurayrah, may Allah be pleased with him, that Medina will be left until a dog or a wolf enters the Prophet's Mosque. Sheikh Al-Albani described a part of this hadith as "munkar" (denounced). This indicates that Medina will become devoid of its inhabitants. As for a man passing by the Prophet's Mosque and saying what you mentioned, no hadith with this meaning has been found.