Yahya related to me from Malik from Ibn Shihab from Ata ibn Yazidal-Laythi from Abu Said al-Khudri that the Messenger of Allah, mayAllah bless him and grant him peace, said, "When you hear the adhan,repeat what the muadhdhin says."
App reference: Book 3, Hadith 2
USC-MSA web (English) reference: Book 3, Hadith 2
Arabic reference: Book 3, Hadith 148