Mishkat al-Masabih 909
‘Abdallah b. Mas'ud said:
When we prayed with the Prophet we said,* “Peace be to God before it is supplicated for His servants; peace be to Gabriel; peace be to Michael; peace be to so and so.” When the Prophet finished he turned his face to us and said: Do not say, “Peace be to God”, for God Himself is Peace. When one of you sits during the prayer he should say, “The adorations of the tongue, acts of worship and all good things are due to God. Peace be upon you, O Prophet, and God’s mercy and blessings. Peace be upon us and upon God’s upright servants (for when he says that it reaches every upright servant in heaven and earth). I testify that there is no god but God, and I testify that Muhammad is His servant and Messenger.” Then he may choose any supplication which pleases him and offer it. 

* I.e. at the tashahhud .

(Bukhari and Muslim.)
وَعَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ قَالَ: كُنَّا إِذَا صَلَّيْنَا مَعَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قُلْنَا السَّلَامُ عَلَى اللَّهِ قبل عباده السَّلَام على جِبْرِيل السَّلَام على مِيكَائِيل السَّلَام على فلَان وَفُلَان فَلَمَّا انْصَرَفَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَقْبَلَ عَلَيْنَا بِوَجْهِهِ قَالَ: «لَا تَقُولُوا السَّلَامُ عَلَى اللَّهِ فَإِنَّ اللَّهَ هُوَ السَّلَامُ فَإِذَا جَلَسَ أَحَدُكُمْ فِي الصَّلَاةِ فَلْيَقُلِ التَّحِيَّاتُ لِلَّهِ وَالصَّلَوَاتُ وَالطَّيِّبَاتُ السَّلَامُ عَلَيْكَ أَيُّهَا النَّبِيُّ وَرَحْمَةُ اللَّهِ وَبَرَكَاتُهُ السَّلَامُ عَلَيْنَا وَعَلَى عِبَادِ اللَّهِ الصَّالِحِينَ فَإِنَّهُ إِذَا قَالَ ذَلِكَ أَصَابَ كُلَّ عَبْدٍ صَالِحٍ فِي السَّمَاءِ وَالْأَرْضِ أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ ثُمَّ لْيَتَخَيَّرْ مِنَ الدُّعَاءِ أَعْجَبَهُ إِلَيْهِ فيدعوه»