In principle and in accordance with the statutory regulations, you must submit a doctor's certificate in the event of illness that is expected to last longer than three days in order to meet your obligation to provide evidence of this. This should be submitted to the employer by the fourth day of your illness at the latest. In principle, however, the employer can also request the "yellow slip" in advance. It can also be agreed in the employment contract that such proof must be submitted on the first day of illness.