Sahih Ibn Khuzaymah

Contrary to the opinion of the person who says that it is not permissible to supplicate with non-Quranic supplications in obligatory prayers.

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that beware, indeed we did not know what to recite in the two rak'ahs (in the Tashahhud) except that we used to glorify Allah, declare His greatness, and praise and extol Him. And indeed Muhammad (peace be upon him) has taught prayers that open the doors of goodness and are comprehensive. Therefore, the Prophet (peace be upon him) said: "When you sit in the Tashahhud after every two rak'ahs, recite these words « التَّحِيَّاتُ لِلّٰہِ وَالصَّلَوَاتُ وَالطَّيِّبَاتُ، السَّلاَمُ عَلَيْكَ أَيُّهَا النَّبِيُّ وَرَحْمَةُ ّٰاللهِ وَبَرَكَاتُهُ، السَّلاَمُ عَلَيْنَا وَعَلَى عِبَادِ َّاللهِ الصَّالِحِينَ، أَشْهَدُ أَنْ لاَ إِلَهَ إِلَّا َّاللهُ وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ »: 'All verbal and pure physical acts of worship are for Allah. O Prophet, peace be upon you and the mercy of Allah and His blessings be upon you. Peace be upon us and upon all the righteous servants of Allah. I bear witness that there is no one worthy of worship except Allah, and I bear witness that indeed Muhammad (peace be upon him) is His servant and Messenger.'" "Then let any one of you choose a supplication he likes and supplicate with it."