Is it true that it was narrated from Umar ibn al-Khattab that he said: "Indeed, I equip the army while I am in prayer," and how could that happen?

Some scholars mention that 'Umar, may Allah be pleased with him, equipped an army while he was praying, and some of them considered this as proof that it is not disliked to ponder over matters of the afterlife during prayer. This is based on a narration that 'Umar prayed a prayer in which he did not recite, and when he was asked about it, he said: "Indeed, I was equipping an army for the Muslims until I reached Wadi al-Qura with them." This was qualified by the condition that it should not distract one from prayer.