Al-Kāfi #1146“Abu ‘Abd Allah (a.s.), has said, ‘Adhan is said with Tartil (articulation slowly) and ‘Iqamah is said quickly.’”
Man Lā Yaḥḍuruh al-Faqīh #870"The takbir (saying 'Allahu Akbar') in the adhan must be pronounced firmly, with clear articulation of the 'ha' and 'alif'“.