Here is what Mandela wrote to Obama: (emphasis mine)
Senator Barack Obama,
Dear Senator Obama,
We join people in your country and around the world in congratulating you on becoming the President-Elect of the United States. Your victory has demonstrated that no person anywhere in the world should not dare to dream of wanting to change the world for a better place.
We note and applaud your commitment to supporting the cause of peace and security around the world. We trust that you will also make it the mission of your Presidency to combat the scourge of poverty and disease everywhere.
We wish you strength and fortitude in the challenging days and years that lie ahead. We are sure you will ultimately achieve your dream making the United States of America a full partner in a community of nations committed to peace and prosperity for all.
N R Mandela