Musannaf Ibn Abi Shayba 3:4018

Waki’ narrated to us, he said: Mis’ar narrated to us, from Zayd al-‘Ammi, from Abu al-Siddiq al-Naji, he said: Ibn Umar saw a group of people who had lain down after the two units of prayer (rak’ahs) of dawn. So he sent someone to them, and he forbade them. They said: "We intend by that the Sunnah." Ibn Umar said: "Return to them and inform them that it is an innovation (bid’ah)."