To say Allahu Akbar in the beginning of every prayer is obligatory, and one of its Rukns, and it is necessary that every letter and the two words are uttered in proper succession. It is also necessary that these two words should be pronounced in correct Arabic. If a person pronounces these words incorrectly, or utters their translation, it will not be valid.
The recommended precaution is that one should not join Takbiratul Ehram of the prayers with any preceding recitations, like, Iqamah or with a Dua which he may be reciting before the Takbir. If a person wishes to join Allahu Akbar with a recitation to follow, like, with Bismillahir Rahmanir Rahim , he should pronounce the "R" of Akbar as Akbaru. However, the recommended precaution is that he should not join it with any other thing in obligatory prayers. It is necessary that when a person pronounces Takbiratul Ehram, his body is steady, if he pronounces Takbiratul Ehram intentionally when his body is in motion, his Takbir is void. A person should pronounce Takbir, Hamd, Surah, Zikr and Dua in such a manner that he should at least hear the whisper. And if he cannot hear it because of deafness or too much noise, he should pronounce them in such a manner that he would be able to hear, if there was no impediment. If a person is dumb, or has some defect in his tongue, rendering him unable to pronounce Allahu Akbar, he should pronounce it in whatever manner he can. And if he cannot pronounce it at all, he should say it is his mind, and should make a suitable sign with his finger for Takbir, and should also move his tongue, if he can. The same rule applies to a person who is born dumb. It is recommended that after the Takbiratul Ehram, a person should say this: Ya muhsinu qad atakal musiu wa qad amartal muhsina an yatajawaza 'anil musiei antal Muhsinu wa anal Musio bihaqqi Muhammadin wa Ali Muhammadin salli 'ala Muhammadin wa Ali Muhammadin wa tajawaz 'an qabihi ma ta'lamu minni..
(O Lord Who are Beneficent! This sinful has come before You and You have ordered the charitable to show indulgence to the sinners. You are Beneficent, and I am a sinner. Bestow Your blessings on Muhammad and his progeny, and pardon my evil acts of which You are aware). It is Mustahab for a person pronouncing the first Takbir of the prayers, and also the Takbirs which occur during the prayers, to raise his hands parallel to his ears. If a person doubts whether he has pronounced Takbiratul Ehram or not, and if he has started Qira'at, he should ignore his doubt. But if he has not recited anything, he should pronounce the Takbir. If after having pronounced Takbiratul Ehram, a person doubts whether he has pronounced it correctly, he should ignore his doubt at any stage.