If you want to tell your business partner that you want out of the business, then you should meet with him and tell him how you feel in a mature and calmly manner. If you aren't able to do so in person, then do so over the phone.
Tell this person what you want. Tell your business partner that you no longer want to be involved with the business. If you are open to explaining why you do not want to be involved anymore, that would be good closure for your partner.
To better assist with the situation it would help to get a full picture of what the business is and why you want to leave the business, instead of a general question.