Havamal

I know that I hung, on a wind-rocked tree, nine whole nights, with a spear wounded, and to Odin offered, myself to myself; on that tree, of which no one knows from what root it springs.

Veit ek at ek hekk vindga meiði á nætr allar níu geiri undaðr ok gefinn Óðni sjálfr sjálfum mér á þeim meiði er manngi veit hvers hann af rótum renn

Havamal: 138, Loddfáfnismál