Musannaf Ibn Abi Shayba 2:117

Abu Bakr narrated to us, he said: Marhum ibn Abd al-Aziz narrated to us, from his father, from Abu al-Zubayr, the mu'adhdhin of Bayt al-Maqdis, who said: Umar ibn al-Khattab came to us and said: "When you give the adhan, then be deliberate (tarassal), and when you give the iqamah, then be quick (hadir)."