Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that when we used to pray behind the Messenger of Allah (peace and blessings be upon him), we would say in the Tashahhud: Peace be upon Jibreel, peace be upon Mika'il, peace be upon Israfil, and peace be upon so-and-so and so-and-so. One day, the Messenger of Allah (peace and blessings be upon him) turned towards us and said: Allah Himself is As-Salam (the Source of Peace), so when you sit in prayer (for Tashahhud), say: «التَّحِيَّاتُلِلَّهِوَالصَّلَوَاتُوَالطَّيِّبَاتُالسَّلَامُعَلَيْكَأَيُّهَاالنَّبِيُّوَرَحْمَةُاللهِوَبَرَكَاتُهُالسَّلَامُعَلَيْنَاوَعَلَىعِبَادِاللهِالصَّالِحِينَ» When you have said these words, the greeting of peace will reach every righteous person in the heavens and the earth and those between them. Then say: «أَشْهَدُأَنلَّاإِلَهَإِلَّااللهُوَأَشْهَدُأَنَّمُحَمَّدًاعَبْدُهُوَرَسُولُهُ» Then ask for whatever (supplication) you wish.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 205