Musannaf Ibn Abi Shayba 8:992

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sharik, from Ibrahim ibn al-Muhajir, from Mujahid, from Ibn Umar regarding the saying of Allah, "The Hajj is [during] well-known months," he said: Shawwal, Dhu al-Qa'dah, and ten [days] of Dhu al-Hijjah.