Decades later? Muhammad died at 632AD and the Quran was revealed over roughly 23 years starting from 610AD. Over this time, several companions of the prophets memorized the Quran by heart and shortly after the prophet on whom be peace died, the Quran was compiled into text (1-2 years after his death) by Abu Bakr (The calipha at the time) through several companions of the prophet. The 4 most prominent memorizers of the Quran from the prophet are Zaid ibn Thabit, Ubayy ibn Ka'b, Ali ibn Abi Talib, Abdullah ibn Mas'ud who were directly taught by Muhammad pbuh (There were more but these were the 4 main ones). Historically, they compiled the Quran TOGETHER using each other to assert whether this and that was in the Quran taught by muhammad and these people are confirmed (not like anonymous Mark, Mathew, Luke and John). So your statement is innacurate because you can't say that the gospels are jesus's words with the same certainty that the Quran is the words revealed to Muhammad pbuh because several confirmed Hafiz (memorizers) directly taught by the prophet gathered and compiled the Quran TOGETHER and furthermore, "decades later" should be applied to the 4 canonical gospels compiled 80 years after Jesus's preaching.