Musannaf Ibn Abi Shayba 3:4304

Waki’ narrated to us, from al-A’mash, from Sulayman ibn Mus-hir, from Kharasha ibn al-Hurr, who said: I saw Umar ibn al-Khattab striking people for [engaging in] conversation after the night prayer (Isha). And he would say: "Stay up during the first part of the night, and sleep during the last part of it."