ربا is undoubtedly haram as per the Qur'an, which says, "
وأحل الله البيع وحرم الربا". This is almost universally translated as "interest" or "usury" in English.
As per the Qur'anic Arabic Corpus, the word ربا is listed with a root of (ر ب و), indicating "to grow". This root is corroborated by Lane's Lexicon, which indicates that it's at least the common (if not the only) root.
Additionally, in the hadith literature, it is well-established that the prophet forbade such transactions that involved "growth," such as from trading goods in unequal amounts, or from loaning out money while expecting more in return; these such transactions are classically referred to as different forms of ربا.
However, according to this answer, the root may in fact be (ر ي ب), indicating "to doubt". Going back to the hadith literature, it is also known that the prophet forbade a number of business transactions in which "doubt" was involved, and is even quoted as having said "So give up riba (ربا) and doubtful things (ريبة)," which at least suggests a relation between the two terms.
Even if the ر ب و root is most prevalent, was (or is) this a known difference of opinion among Islamic scholars? What evidence is there on the actual root of the word ربا, and how can we be sure (one way or the other) which one is correct?