It is generally accepted as fact that when the Mongols under Hulagu sacked Baghdad, capital of the Abbasid Caliphate, they destroyed all (36 libraries) including the famed House of Wisdom, and either burned, looted or threw all the prized books in the Tigris, however what are the "primary sources" of actual witnesses for these tales?
Contemporary accounts state Mongol soldiers looted and then destroyed mosques, palaces, libraries, and hospitals. Priceless books from Baghdad's thirty-six public libraries were torn apart, the looters using their leather covers as sandals. Grand buildings that had been the work of generations were burned to the ground. The House of Wisdom (the Grand Library of Baghdad), containing countless precious historical documents and books on subjects ranging from medicine to astronomy, was destroyed. Claims have been made that the Tigris ran red from the blood of the scientists and philosophers killed. Tales of the destruction of books – tossed into the Tigris such that the water turned black from the ink - seem to originate from the 14th century
Wikipedia: Siege of Baghdad (1258)