It depends on the situation: If you invite someone over to your house and you want to make them feel comfortable and at home, you can say "اتفضل ,البيت بيتك" pronounced as "etfadal, el bet betak". If you want someone to feel comfortable in any place you say : اتفضل او خد راحتك as others have previously pointed out. Pronounced " etfadal, khod rahtak. The word "استريح" is correct but other phrases are more commonly used.