Question:

If after hearing the footsteps of someone or the door of the masjid opening, the Imam lengthens the qirat, ruku or last sitting, what will be the ruling of such a salah? An Imam has said the salah becomes invalid.

Answer: حامدا ومصليا ومسلما

Lengthening the qirat, ruku or last sitting to allow a person to join the congregation will be makrooh tahrimi (highly reprehensible) in the scenario where the imam recognizes the person and delays the prayer due to his acquaintance with him.

However, even in this situation the salah is not invalid.

If the Imam did not recognize him, rather he only did such to help a person catch the rakat or salah, then there is no blame in that, rather he will be rewarded.

In Durr Mukhtaar the following is mentioned:

وكره تحريما إطالة ركوع أو قراءة لإدراك الجائي (أي إن عرفه وإلا فلا بأس به ولو أراد التقرب إلى الله ) (شامي 1:365-366)

However one should remember that this lengthening should not be more than one or two tasbeehs.

And Allah ta’aalaa knows best.
Hazrat Mufti Ahmad Khanpuri Saheb (May Allah preserve him)
Mahmoodul Fataawaa 1/462