While playing with my buddy, he played Chemister's Insight. He drew his 2 cards and so the spell was resolved. He then switched out one of his mana that he had tapped for Chemister's Insight so that he could play another card. He had tapped all of his blue mana then switched one out for a red after the spell resolved so that he could play a blue card. I called him out for it and he said that the mana still legally payed for Chemister's Insight so it doesnt matter. Does anyone know the official rulings on this?
I agree but he wants to see an actual rule. I'm having trouble finding it.