(a) Al-Sha'bi (may Allah have mercy on him) narrates that Abu Musa al-Ash'ari (may Allah be pleased with him) said to Umar bin al-Khattab (may Allah be pleased with him): O Commander of the Believers, did you recite inwardly? He said: No. Then he ordered the mu'adhdhins, and they gave the adhan and the iqamah, and he led them in the prayer again.
These narrations from Ibrahim and al-Sha'bi are mursal, as al-Shafi'i said; and the narration of Abu Salamah, even though it is mursal, is the more authentic of the mursal reports, and he narrated it in Madinah, the place where the incident occurred, where, as al-Shafi'i said, no one objected to it. However, the hadith of al-Sha'bi has been narrated with a connected chain through another route.
(b) Repeating the prayer is closer to the Sunnah regarding the obligation of recitation, and recitation, like the other pillars of the prayer, is not waived through forgetfulness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3983