Is there any Shar’i evidence for alerting a person in the first row to move to the second row when there is no space, and is the prayer of someone who retreats and takes many steps valid, and is the act of the alerter correct and sunnah?

The prayer of someone who steps back to stand with the person who alerted them is valid, provided they do not take more than two steps. If they take more than that, some jurists, such as the Shafi'is, consider their prayer void, as they believe that many consecutive steps invalidate the prayer. It is safer for someone who walked more than five steps to repeat their prayer, and it is said to be safer, not obligatory, because the Malikis permit walking in prayer to fill gaps, even if it exceeds two rows. The action of the person who stayed behind is correct if they did not know that the walking of the person who alerted them would invalidate their prayer. Jurists differ on what should be done in this situation, and most of them permit pulling an individual from the صف (row) to stand with them.