Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The key to prayer is ablution, its tahrim (entry into its sanctity) is the takbir, and what makes things lawful again is turning with the salam; in every two rak'ahs there is a salam; and the prayer is not complete unless Surah al-Fatihah and something else besides it is recited."
Abu Abd al-Rahman (may Allah have mercy on him) says: I asked Abu Hanifah (may Allah have mercy on him): What is meant by "in every two rak'ahs there is a salam"? He said: By it the tashahhud is meant.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3973
Hadith Gradingمحدثین:صحيح لغيرهٖ
Hadith Takhrij«صحيح لغيرهٖ۔ وله شاهد رواه الطبراني في مسند الشامين 2/289»