I have an etexts version of The Inferno and have searched the entire text, but I cannot find the quotation you want to know about. Rather than locating a direct quotation, could the question be more like "Where does one find the concept or theme of honoring commitments in The Inferno?" I'ved cited the Enotes study guide to the themes in this text.

Or does your question refer to the "uncommited souls" whom Dante writes about in Canto III? The uncommitted are souls who are neither good nor evil--they never took a side.

This miserable mode

Maintain the melancholy souls of those

Who lived withouten infamy or praise.

Commingled are they with that caitiff choir

Of Angels, who have not rebellious been,

Nor faithful were to God, but were for self.

(from the Longfellow translation)

See the Enotes summary (linked below). I've also included a link to the site where you can find the Longfellow translation of The Inferno.