I downloaded a song from itunes on my iphone how do i get it for FREE onto my ipod touch?

I downloaded a song from itunes on my iphone how do i get it for FREE onto my ipod touch i went to download it on my itunes tab on my ipod after i purchased on my iphone but it says do u wanna buy again and i dont wanna bhy again iwant it free. When u are using the app store u can transfer for free it will ask would u like to download for free but it didnt with itunes