Musannaf Ibn Abi Shayba 2:6

Abu Bakr narrated to us, he said: Yazid ibn Harun narrated to us, he said: Sulayman al-Taymi informed us, from Habib ibn Qays, from Ibn Abi Mahdhura, from his father, that he used to lower his voice in the Adhan once, once. Until when he reached his saying, "I bear witness that Muhammad is the Messenger of Allah," he would return to his saying, "I bear witness that there is no god but Allah," and he would raise his voice with it twice, twice. Until when he reached "Come to prayer," he would say, "Prayer is better than sleep," in the first Adhan for the dawn prayer.