A:
First:
it is Mashru‘ (islamically acceptable) for an Imam to say: “Straighten up and be straight,” as it was authentically reported that the Messenger of Allah (peace be upon him) said,
“Straighten your rows, for straightening the row is part of perfecting the Salah”
It was narrated in "Sunan
Abu Dawud" that, when establishing the Salah, the Messenger of Allah (peace be upon him) would turn to his right and say:
“Straighten up! Straighten your rows!” and then he would turn to his left and say, “Straighten up! Straighten your rows!”
Another narration reads,
“Complete the front row, then the one that comes next, and if there is any incompleteness, let it be in the last row.”
The Hadith narrated by
Muslim reads,
“Straighten the row in Salah, for straightening the row is part of performing the Salah well.”
In the Hadith narrated by
Abu Dawud,
“Straighten the rows, keep your shoulders in line with one another, close the gaps, be pliable (movable) in the hands of your brethren, do not leave any gaps for Satan. Anyone who joins a row, Allah will join him (to His Mercy); and anyone who severs a row, Allah will sever him (from His Mercy).”
Second:
as for the exhortation that the Imam offers before Takbirat-ul-Ihram (saying: "allahu akbar [Allah is the Greatest]" upon starting Prayer) namely his saying: (Part No. 6; Page No. 328) “Pray like one bidding farewell! Attend with your heart your Salah (prayer) and show submissive humility in it!” and the like, this is not Mashru‘; rather, it is Bid‘ah (innovation in religion).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.