All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is Sunnah for the person who intends to perform Ablution to say Bismillah at the beginning of ablution.
The Prophet said: “There is no prayer for one who does not have ablution, and there is no ablution for one who does not mention the Name of Allah (before it).” [Abu Daawood]
At the end of ablution, one should say: “Ashhadu an Laa Ilaaha Illa Allah Wa Ashhadu Anna Muhammadan Abduhu Wa Rassooluhu.”
The Prophet also said: “If anyone amongst you performs ablution, and completes it well and then says: 'Ashhadu an laa ilaaha illa Allah, wa Ashhadu anna Muhammadan 'abduhu wa rasooluhu' (I bear witness that none has the right to be worshipped but Allah, and I bear witness that Muhammed is His servant and His Messenger), eight gates of Paradise will be opened to him and he may enter through whichever one he wants.”’[Muslim]
Another narration reads: “O Allah! Let me be one of the repenting ones and let me be one of the purified ones.” [At-Tirmithi
For more benefit on the detailed description of ablution, please refer to Fataawa 82845 and 83164.
Allah knows best.