Musannaf Ibn Abi Shayba 2:22

Abu Bakr narrated to us, he said: Ali bin Hashim narrated to us, from Ibn Abi Layla, from Amr bin Murrah, from Abd al-Rahman bin Abi Layla, who said: Abdullah bin Zayd al-Ansari, the mu'adhdhin of the Prophet (may Allah bless him and grant him peace), used to double the phrases of the Adhan and the Iqamah.