Imam al-Shafi'i (may Allah have mercy on him) said: The tasmiyah when slaughtering an animal is 'Bismillah' (In the name of Allah); if one adds something of the remembrance of Allah after that, the addition is good. And regarding sending blessings upon the Messenger of Allah (peace and blessings be upon him) while slaughtering an animal, he said: I like this too; indeed, sending blessings upon the Messenger of Allah (peace and blessings be upon him) in all circumstances is even better, because the remembrance of Allah and sending blessings upon him (peace and blessings be upon him) are worship, and the one who says them is rewarded.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19175