Sahih Bukhari, Volume 9, Book 88, Number 235:
Narrated Abu Huraira:
Allah's Apostle said, "Soon the river "Euphrates" will disclose the treasure (the mountain) of gold, so whoever will be present at that time should not take anything of it." Al-A'raj narrated from Abii Huraira that the Prophet said the same but he said, "It (Euphrates) will uncover a mountain of gold (under it)."
Sahih Muslim, Book 41, Number 6918:
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: The Last Hour would not come before the Euphrates uncovers a mountain of gold, for which people would fight. Ninety-nine out of each one hundred would die but every man amongst them would say that perhaps he would be the one who would be saved (and thus possess this gold).
An indication that they will know this hadith about the Gold in Euphrate.