1 Samuel 17:39-40

39David tied Saul's sword on top of the armour. Then he tried to walk. But he had not worn heavy armour like that before. So David said to Saul, ‘I cannot wear all this armour to fight. I have not used armour before.’ So David took it all off.

40Instead, David picked up his shepherd's stick. He picked up five round stones from the stream. He put them in the pocket of his shepherd's bag. He held his sling in his hand. Then he walked towards the Philistine soldier.