I have seen many claims about how the Zamzam well will never dry out. But, is there any backup for this from the Qur'an and Sunnah, implicit or explicit?
As per Islamweb, there is an Ahmad hadith that says this:
Ibn 'Abbaas, may Allah be pleased with him, narrated that the Prophet sallallaahu `alayhi wa sallam ( may Allah exalt his mention ) said: "...May Allah bestow his mercy upon her (Hagar, the mother of Ismaa’eel), had she let go (of the water of Zamzam) it would have become a spring (rather than a well) whose water shall never dry." [Ahmad]
Note: There's a similar hadith in Bukhari but it does not contain the "whose water shall never dry" portion.
I currently do not yet know of the exact Ahmad reference (book, hadith #). If someone can help find the specific reference, that would be great!