(a) Abdullah (may Allah be pleased with him) narrates: The tashahhud is what completes the prayer.
(b) It has been narrated to us from Umar bin al-Khattab (may Allah be pleased with him) that he said: No prayer is valid without the tashahhud.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3964