Musannaf Ibn Abi Shaybah 4056 — Those who used to say: if one forgets

Hammam said: Once Sayyiduna Umar (may Allah be pleased with him) led the Maghrib prayer and did not recite in it. When he finished the prayer, the people said to him: "O Commander of the Believers! You did not recite." Sayyiduna Umar said: "During the prayer I was thinking to myself about a caravan of camels that I had dispatched from Madinah; I kept on equipping it in my mind until it entered al-Sham." Then he repeated the prayer and the recitation.