"He sang of the nine days that the all-father hung from the world tree"

In the story of Runatal, found in the Havamal, Odin is described as sacrificing himself to himself by hanging from the world tree Yggdrasil.

Pierced by his own spear, he remained there for nine days and nights so that he would learn power over the nine worlds. In Norse magical practices, there are nine realms of existence. Odin also learns nine (later eighteen) magical songs and eighteen magical runes.