Musannaf Ibn Abi Shayba 3:1629

Abu Bakr narrated to us, he said: Abdullah bin Numayr narrated to us, from Ubaydullah bin Umar, from Muhammad bin Ibrahim, from Abu Salamah, who said: Umar prayed the Maghrib prayer and did not recite [al-Fatiha]. When he finished, the people said to him, "You did not recite." He said, "How was the bowing and the prostration? Was it complete?" They said, "Yes." He said, "There is no harm. I was speaking to myself about a camel that I was preparing with its saddles and its packs."