Musannaf Ibn Abi Shayba 2:222

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Hisham, from Yahya, he said: It was narrated to me that the Messenger of Allah (may Allah bless him and grant him peace) said: "If the people knew what is in the Adhan, they would have vied for it." He said: And it used to be said: "Hasten to the Adhan, and do not hasten to the Iqamah."