Rationale: A standard work-up of stroke patients to identify a cause of stroke consists of Computed Tomography (CT), CT angiography of head and neck arteries, transthoracic echocardiography, 12-lead 10-seconds electrocardiogram (ECG), blood tests and 24-hour ECG monitoring. A stroke is called "cryptogenic" if no cause can be determined after standard work-up. Current detection of atrial fibrillation (AF) after stroke is 2.0%. Recent studies revealed that prolonged monitoring yields higher percentages of detected AF (12.0%). Devices used in these trials suffer from drawbacks. The investigators' hypothesis is that a new, smartphone compatible device, producing a single lead ECG (single lead ECG device), can be used for prolonged ECG monitoring in cryptogenic stroke patients.