Al-Matalib al-'Aliyah 669 — Forgetfulness in prayer

It is narrated from Damdam bin Jaws that I came to Sayyiduna Abu Hurayrah and Sayyiduna Abdullah bin Hanzalah (may Allah be pleased with them both) while the two of them were sitting in the mosque at the time the sun passed its zenith. Sayyiduna Abdullah bin Hanzalah (may Allah be pleased with him) related: "Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) led us in the Maghrib prayer and forgot to recite in the first rak'ah. So when he stood for the second rak'ah he recited Surah al-Fatihah and a surah, then he again recited Surah al-Fatihah and a surah (to make up for the first rak'ah), then continued his prayer until he completed it, then he made the two prostrations of forgetfulness."
Imam al-Harith bin Abi Usamah narrated this in his Musnad; this report is sahih (authentic) and indicates the method of making up for forgetting the recitation in the prayer.