Isaiah 1:16 Commentary: Rashi & Ibn Ezra

Wash, cleanse yourselves Voweled with a ‘patach,’ the imperative form, since it is derived from רְחַץ, but רָחֲצוּ, [in the past tense, is voweled with a ‘kamatz’ because it is derived from רָחַץ].

Wash. Wash your hands of the blood they have shed.

Wash, cleanse yourselves, remove, learn, seek, strengthen, perform justice, plead, go Ten exhortations of the expression of repentance are [listed] here, corresponding to the Ten Days of Penitence and to the ten verses of Kingship, Remembrances, and Shofaroth [in the musaf service of Rosh Hashanah].