Abdullah ibn Mas'ud reported: We were saying in prayer behind the Messenger of Allah, "Peace be upon Allah; peace be upon so-and-so..." So the Messenger of Allah, peace and blessings be upon him, said to us one day, Verily, Allah Himself is Peace. So when any of you sit in prayer, let him say: Greetings, prayers, and good things are for Allah; peace be upon you, O Prophet, and the mercy of Allah and His blessings; peace be upon us and upon the righteous servants of Allah. If you say that, it will reach every righteous servant in the heavens and on earth. Then say: I bear witness that there is no God but Allah, and I bear witness that Muhammad is His servant and His Messenger; then you may ask for whatever you wish."
Source: Sahih Muslim 402
Grade: Sahih (authentic) according to Imam Muslim
عَنْ عَبْدِ اللَّهِ قَالَ كُنَّا نَقُولُ فِي الصَّلَاةِ خَلْفَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ السَّلَامُ عَلَى اللَّهِ السَّلَامُ عَلَى فُلَانٍ فَقَالَ لَنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ذَاتَ يَوْمٍ إِنَّ اللَّهَ هُوَ السَّلَامُ فَإِذَا قَعَدَ أَحَدُكُمْ فِي الصَّلَاةِ فَلْيَقُلْ التَّحِيَّاتُ لِلَّهِ وَالصَّلَوَاتُ وَالطَّيِّبَاتُ السَّلَامُ عَلَيْكَ أَيُّهَا النَّبِيُّ وَرَحْمَةُ اللَّهِ وَبَرَكَاتُهُ السَّلَامُ عَلَيْنَا وَعَلَى عِبَادِ اللَّهِ الصَّالِحِينَ فَإِذَا قَالَهَا أَصَابَتْ كُلَّ عَبْدٍ لِلَّهِ صَالِحٍ فِي السَّمَاءِ وَالْأَرْضِ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ ثُمَّ يَتَخَيَّرُ مِنْ الْمَسْأَلَةِ مَا شَاءَ
402 صحيح مسلم كِتَاب الصَّلَاةِ باب التَّشَهُّدِ فِي الصَّلَاةِ