After the Prophet (peace be upon him), it was not Abu Bakr or Umar (may Allah be pleased with them), as some ignorant people have claimed that it is possible that Abu Bakr as-Siddiq or Umar al-Faruq (may Allah be pleased with them) ordered Bilal (may Allah be pleased with him) to do this.
Sayyiduna Anas (may Allah be pleased with him) narrates that the Companions searched for something by which they could announce the time of prayer, so that they could call the Adhan. He says that Sayyiduna Bilal (may Allah be pleased with him) was commanded to say the words of the Adhan twice each and the words of the Iqamah once each.