Musannaf Ibn Abi Shayba 29:131

Muhammad ibn Bishr narrated to us, he said: Hisham ibn Sa’d narrated to us, he said: ‘Urwah ibn Ruwaym narrated to me, from al-Qasim, from ‘Abdullah ibn ‘Umar, who said: I came to ‘Umar when he arrived in al-Sham, and I found him taking a midday nap in his tent. So I waited for him in the tent, and I heard him when he stirred from his sleep, and he was saying: "O Allah, forgive me for my return from the expedition of Sargh," meaning when he returned because of the plague.