Musannaf Ibn Abi Shayba 3:1635

Abu Mu'awiyah narrated to us, from al-A'mash, from Ibrahim, from Hammam, who said: Umar prayed the Maghrib prayer and did not recite in it. When he finished, they said to him, "O Commander of the Faithful, you did not recite." He said, "I was speaking to myself while I was in the prayer about a camel I was preparing from Medina, and I did not cease preparing it until it entered al-Sham." He said: Then he repeated the prayer and the recitation.