Abu al-Zubayr, who was the muezzin of Bayt al-Maqdis, said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said to me: When you give the adhan, give it slowly, and when you give the iqamah, say it quickly.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2010