Musannaf Ibn Abi Shayba 2:17

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, he said: He used to say: The Iqamah is once, once. But when he says "Qad qamat al-salah" (the prayer has been established), he says it twice.