Interesting read.....
At the time of war as Bhishma who was on the side of Kaurava was accused by Duryodhana for doing any harm to Pandavas and favoring them. On hearing this Bhishma became very angry and in his anger, he took five gold arrows filled it with mantras. He vowed that the Pandvas would die before the Sunset. On the other day Duryodhana suspected that Bhisma would never do anything wrong with them so, he took those arrows with him. Lord Krishna reminded Arjun that Duryodhana had promised him of boon and he should ask him for five golden arrows .When Arjun asked him of the arrows Duryodhana was shocked and asked him how did he know about those arrows then Arjun replied “Krishna”.