Bismillahir-Rahmaanir-Raheem
As-Salaamu alaykum wa rahmatullaahi wa barakaatuhu, In reality there is no formula to mentioning the intention in salah, it is a religious innovation and as such it is forbidden to do as our beloved Messenger [sallallahu alayhi wa salam] mentioned in many ahadeeth[plural of hadeeth] the forbiddance of innovations when he [alayhi salam] said,"man amila amalan laysa alayhi amrunaa fahuwa raddun" whoever does an act that is not in agreement with this affair of our [this religion] shall have it rejected" in another narration he [alayhi salam] said on the authority of Aa'isha [radhiya Allahi anha] meaning Aa'isha narrated this hadeeth "man ahdathaa fee amrinaa hadha maa laysa minhu fahuwa raddun" Whoever introduces into this affair of ours [our religion] that which isn't from it shall have it rejected." Both of these authentic narrations can be found in Saheeh Muslim and Saheeh Bukhaaree. So it is not permissible to do an act that is not legistlated by Allah and His Messenger [sallallahu alayhi wa sallam] and what I mean by not permissible I mean it is HARAAM forbidden not to mention that if a person were to do so anyway then he shall be punished as Allah says, "falyahdharilladheena yukhaalifoona an amrihi an tuseebahum fitnah aw yuseebahum adhaabun aleem" "Let those beware who oppose the messenger[sallallahu alayhi wa sallam] commandments lest some fitnah befall them or some severe punishment." 24:63 So in this ayah it is clear that opposing his commandments involes one falling into trials or receiving an adhaabul aleem [a severe punishment] as the messenger said, "alaykum bi sunnati, wa sunnatul khulaafah ar-rashideen" so stick to my sunnah and the sunnah of the rightly guided khaleefs after me" We were forbidden from religious innovates and commaded to stick to his sunnah and the sunnah of the khulafah. And no one from amongst the companions ever practiced mentioning the niyyah for any act of ibaadah except Hajj and Umrah. Lastly your intention for prayer is already present with you once you make your wudoo for the salah in its proper time, there is no need to pronounce anything because as the people of the past from the righteous used to say, "mahaalatun niyyah fil qalb" The abode of the niyyah is in the heart" Wa Billahit-Tawfeeq!