A 68-year-old man with hypertension and type 2 diabetes attends the emergency department. Two hours ago, while reading the paper, he suddenly developed right hand weakness and slurred speech, which lasted about 20 minutes and then resolved completely. He had no headache, no visual disturbance and no jerking movements. He is now asymptomatic. Pulse is irregularly irregular at 88/min, blood pressure is 164/92 mmHg, capillary glucose is 7.4 mmol/L (3.0–6.0), and neurological examination is entirely normal.

What is the most likely diagnosis?