Mentioning the criterion for shortening the prayer when leading people.

The Shaykh : But what is the measure of the shortening required? Its measure is the prayer of the Prophet (peace and blessings be upon him); one should lead the people in prayer just as the Messenger of Allah (peace and blessings be upon him) used to lead them. For this reason, Anas said: So shortening does not mean that a person clatters through it like a crow, but rather that he prays as the Prophet (peace and blessings be upon him) used to pray. So what do you say about a man who led the people in the festival prayer and recited in the first rak'ah and in the second rak'ah Surat al-Qamar ? Is this prolonged or shortened? It is shortened, because it is part of the Sunnah to recite in the festival prayer Surat in the first rak'ah and Surat in the second rak'ah, and sometimes and al-Ghashiyah. And in the Friday prayer, sometimes Subha and al-Ghashiyah, and sometimes al-Jumu'ah and al-Munafiqun. Thus, the two prayers coincide in two surahs and differ in two surahs. Yes, that is right. Now, what is it? So it is incumbent upon a person, if he is in charge of something, to observe the conditions of those under his care. The Messenger of Allah (peace and blessings be upon him), when he heard the crying of a child, would shorten his prayer for fear that his mother would be tried and her heart would be distracted. This is part of complete care, because this shortening was due to a temporary circumstance and he did not always consider the mother. But when this thing occurred and her child cried, he (peace and blessings be upon him) shortened the prayer. From this, the scholars (may Allah have mercy on them) derived that it is incumbent upon the imam, if he senses that someone has entered the prayer while he is bowing, to wait for him. For example, if one person enters the mosque and you hear him while you are bowing in congregation, wait for him, but on the condition that you do not burden the ma'mums with you, because they are more deserving of consideration than the one who enters late.