have someone's best interest(s) at heart
to make decisions based on someone's best interests.
1-I know she was only doing what would benefit her, but she said she had my best interests at heart.
2-Never kid yourself that they [politicians] have your interests at heart, especially when they say such things as “I'm with you, shoulder to shoulder,” because at the first sign of trouble , you'll find yourself standing next to the invisible man!
Ref: http://idioms.thefreedictionary.com/have+best+interest+at+heart
And there is another expression :in one's interest -به نفع/ صلاح كسي
(Also, in the interest of one; in one's own interest; in one's best interest. ):For one's benefit or advantage, as in :
1-It's obviously in their interest to increase profits.
2-Is this policy in the interest of the towns people?
3-I suspect it's in your own best interest to quit now.
Ref: http://dictionary.reference.com/browse/in+one's+interest