735 ILCS 5/9-209
A landlord or his or her agent may, any time after rent is due, demand payment thereof and notify the tenant, in writing, that unless payment is made within a time mentioned in such notice, not less than 5 days after service thereof, the lease will be terminated. If the tenant does not pay the rent due within the time stated in the notice under this Section, the landlord may consider the lease ended and commence an eviction or ejectment action without further notice or demand. A claim for rent may be joined in the complaint, including a request for the pro rata amount of rent due for any period that a judgment is stayed, and a judgment obtained for the amount of rent found due, in any action or proceeding brought, in an eviction action under this Section.
Notice made pursuant to this Section shall, as hereinafter stated, not be invalidated by payments of past due rent demanded in the notice, when the payments do not, at the end of the notice period, total the amount demanded in the notice. The landlord may, however, agree in writing to continue the lease in exchange for receiving partial payment. To prevent invalidation, the notice must prominently state:
"Only FULL PAYMENT of the rent demanded in this notice will waive the landlord's right to terminate the lease under this notice, unless the landlord agrees in writing to continue the lease in exchange for receiving partial payment."
Collection by the landlord of past rent due after the filing of a suit for eviction or ejectment pursuant to failure of the tenant to pay the rent demanded in the notice shall not invalidate the suit.
735 ILCS 5/9-209