If you believe the latest scientific study, prayer doesn't heal. William Saletan offers 17 hypotheses to explain this negative result. For #15, "God doesn't participate in studies," I'd throw in a few verses from Matthew 4.

Then the devil took [Jesus] to the holy city and had him stand on the highest point of the temple. "If you are the Son of God," he said, "throw yourself down. For it is written:

" 'He will command his angels concerning you, and they will lift you up in their hands, so that you will not strike your foot against a stone.'"

Jesus answered him, "It is also written: 'Do not put the Lord your God to the test.'"

See also Matthew 12.

Then some of the Pharisees and teachers of the law said to [Jesus], "Teacher, we want to see a miraculous sign from you." He answered, "A wicked and adulterous generation asks for a miraculous sign! But none will be given it except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth...."