Sheikh Sâlim al-Qarnî
You may say “Bismillâh” (In the namer of Allah) or “Bismillâh wa Allah Akbar” (In the name of Allah; Allah is the greatest). It is permissible to add: “Rabbanâ taqabbal minnâ” (Our Lord, accept this from us).
Since you can suffice with “Bismillah”, then the slaughtering will still be correct if you add “al-Rahmân al-Rahîm” (the Beneficent, the Merciful). However, it is preferrable not to say something that is not established by the Sunnah.
Source: Islam Today