Musannaf Ibn Abi Shayba 3:5294

Abu Muhammad Abdullah ibn Yunus narrated to us, he said: Baqi ibn Makhlad—may Allah have mercy on him—narrated to us, he said: Abu Bakr narrated to us, he said: Yahya ibn Sa’id al-Qattan narrated to us, from Muhammad ibn Yusuf, that al-Sa’ib informed him: That Umar gathered the people under Ubayy and Tamim, and they would pray eleven rak'ahs, reciting the hundreds (al-mi'in) — meaning in Ramadan.