The modification of agreement clause in a contract states that the agreement may only be modified in writing. As important as it is to have your contract in writing, it is equally important to require any modifications — additions, deletions, or changes in wording — to be in writing. You don't want a conversation or telephone call that you had with the other side to modify your written contract.
Here is a sample clause:
Modification of Agreement. This Agreement may be supplemented, amended, or modified only by the mutual agreement of the parties. No supplement, amendment, or modification of this Agreement shall be binding unless it is in writing and signed by all parties.