The American Bar Association’s Model Rule 1.5(b) governs how lawyers state their fees. “The scope of the representation and the basis or rate of the fee and expenses for which the client will be responsible shall be communicated to the client, preferably in writing, before or within a reasonable time after commencing the representation …” The exception is a regularly represented client charged on the same basis.
For a contingent fee, writing is not only preferred. Rule 1.5(c) says: “A contingent fee agreement shall be in a writing signed by the client.”