The call to prayer is done in pairs: like the adhan; as reported by Abdul Rahman bin Abi Layla, may Allah have mercy on him, who said: "Our companions of Muhammad, peace be upon him, told us that Abdullah bin Zayd, when he saw the adhan, came to the Prophet, peace be upon him, and informed him. The Prophet said: Teach it to Bilal. So Bilal stood up and called the adhan in pairs, and he established the prayer in pairs, and he sat in a sitting position." This is found in Sahih Ibn Khuzaymah 1: 196, and Al-Ahad wal-Mathani 3: 476, and Sharh Ma'ani Al-Athar 1: 131, and its chain is authentic, as noted in I'la Al-Sunan 2: 99. And from Al-Sha'bi from Abdullah bin Zayd Al-Ansari, may Allah be pleased with him: "I heard the adhan of the Messenger of Allah, peace be upon him, and his adhan and iqamah were in pairs." This is in Musnad Abu Awanah 1: 276, and it is a strong mursal, as mentioned in I'la Al-Sunan 2: 100-101. See: Bada'i Al-Sanai 1: 147.