(a) It is narrated from Sayyiduna Ali (may Allah be pleased with him) that he (peace and blessings be upon him) said: "The key to the prayer is purification (wudu), what makes things forbidden in it is the takbir, and what makes them lawful again is the salam."
(b) A narration on this from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) has already passed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2962