It is reported from Ibrahim that Abu Musa al-Ash'ari (may Allah be pleased with him) said: "O Commander of the Believers, did you recite inwardly?" He said: "No." He said: "Then you have certainly not recited anything." So Umar (may Allah be pleased with him) repeated the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3982