"Sine" is a homophone of "sign". In mathematics, sine refers to a trigonometirc function that shows the relationship between two sides in a right triangle. A sign is a signal that gives information to a person. After prayer, people look "for a sign" for guidance on the prayer just made. This joke is playing on the word "sine" and its homophone "sign". 
I always prayed before my trigonometry tests . I was hoping for a sine from above .