"The Prophet would open his Salat with (Bismillahir-Rahmanir-Rahim)."
Jami` at-Tirmidhi Book 2, Hadith 97
← Previous Hadith "While I was praying, I said: Bismillahir-Rahmanir-Rahim "In the Name of Allah, the Merciful, the Beneficent." My father heard me and said: "O my son this is a newly invented matter, beware of the newly-invented.'" He (Ibn Abdullah) said: "I have not seen any one of the Companions of Allah's Messenger who hated a newly invented matter in Islam more than him. And he said: 'Ihave performed Salat with the Prophet, and with Abu Bakr, and Umar, and with Uthman. I did not hear any one of them saying it. So do not say it. When you are performing Salat say: Al-Hamdu lilahi Rabbil-Alamin "All praise is due to Allah the Lord of all that exists.'"

Jami` at-Tirmidhi Book 2, Hadith 96 Next Hadith → "Allah's Messenger, Abu Bakr, Umar and Uthman opened the Salat with (Al-Hamdu-lillahi rabbil-alamin)."

Jami` at-Tirmidhi Book 2, Hadith 98