Musannaf Ibn Abi Shayba 3:3552

Yahya ibn Abi Bukayr narrated to us, he said: Ibrahim ibn Nafi’ narrated to us, from Ibn Abi Najih, from Mujahid, that ‘Abdullah ibn ‘Umar would not pray them except in Ramadan, meaning between the Maghrib and the ‘Isha’ [prayers].