Reference : Hisn al-Muslim 23

وَأَنا أَشْـهَدُ أَنْ لا إِلـهَ إِلاّ اللهُ
وَحْـدَهُ لا شَـريكَ لَـه ، 
وَأَنَّ محَمّـداً عَبْـدُهُ وَرَسـولُه،
رَضيـتُ بِاللهِ رَبَّاً ، 
وَبِمُحَمَّـدٍ رَسـولاً
وَبِالإِسْلامِ دينَـاً
Wa 'anā 'ash-hadu 'an lā 'ilāha 'illallāhu 
waḥdahu lā sharīka lahu 
wa 'anna Muḥammadan 'abduhu wa rasūluhu, 
raḍītu billāhi rabban, 
wa bi Muḥammadin rasūlan 
wa bi 'l-islāmi dīnan.

I bear witness that none has the right to be worshipped but Allah
alone, Who has no partner, 
and that Muhammad is His slave and His Messenger. 
I am pleased with Allah as my Lord, 
with Muhammad as my Messenger 
and with Islam as my religion.1

[To be recited in Arabic after the mu'adh-dhin's tashahhud or the words of affirmation of Faith] 2

Reference:
1. Muslim 1/290.
2. Ibn Khuzaymah 1/220.

Chapter: Concerning the athan (the call to prayer)