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بِسْمِ اللَّهِ، وَعَلَى سُنَّةِ رَسُولِ اللَّهِ
Bismillah, wa'ala sunnati rasulillah
In the name of Allah, and according to the Sunnah of the Messenger of Allah.
Source Sunan Abī Dāwūd 3213 Ṣaḥīḥ
Compiler cites Abū Dāwūd 3/314, with a ṣaḥīḥ chain; Aḥmad has 'wa ʿalā millati rasūli-llāh' (upon the religion of the Messenger of Allah), also with a ṣaḥīḥ chain