Does the idea of praying 5 times a day come from the Qur'an?

One of my maternal uncles argued that, since the Qur'an does not explicitly say anything about praying 5 times a day, it may be considered as optional/less important. What argument can I place against his view? As the argument is coming from a Qur'an-only perspective, I am particularly interested in arguments that don't rely on hadiths.