The river Humber.
For by the river that whylome was hight The ancien Abus ... [was from] Their chieftain, Humber, named aright.
And Drayton, in his Polyolbion, 28, says:
For my princely name. From Humber, king of Huns, as anciently it came.
See Geoffrey's Chronicles, Bk. ii. 2.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894