بِسْمِ اللَّهِ وَعَلَى سُنَّةِ رَسُولِ اللَّهِ
With the Name of Allah and according to the Sunnah of the Messenger of Allah.
Bismillaahi wa 'alaa sunnati Rasoolillaahi.
Abu Dawud 3/314 with an authentic chain. Ahmad also recorded it with the wording: With the Name of Allah, and according to the religion of the Messenger of Allah. Its chain is also authentic.