The main character gets stabbed by his childhood friend who is a sage, because he sees the sage with the other childhood friend who is a hero and the main character floats down a river, sees a dragon and then reincarnates.
This is I Am a Villager, What About It?, aka Murabito desu ga Nani ka?.
Born in the remote countryside, Ryuto’s childhood friends were a cheat of a female hero and a wise man. One day at the age of 16, he was killed by a wise man, who was his childhood friend. Soon, Ryuto was reincarnated as a villager. Get the “effort cheat” and the “knowledge cheat”! Although the status is the weakest, he is capable of even saving a hero! This is a story about Ryuto in the adventure world becoming the strongest villager on the ground!
The description is probably machine translated; "wise man" is the sage.
He is poisoned by his friend, the sage, not stabbed
and is then dumped into the river, so the sage can continue to be with the hero. While floating along, he meets a dragon,
who handles the reincarnation.