After completing Wudu
أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ
Ash-hadu allaa ilaaha illallaahu wahdahu laa shareeka lah, wa ash-hadu anna Muhammadan 'abduhu wa rasooluh.
I testify that there is no god but Allah alone with no partner, and I testify that Muhammad is His servant and messenger.
Source: Sahih Muslim 234