Nasai

1400. Commentary: This applies when there is no empty space in the rows ahead. If there is an empty space ahead but it cannot be reached without stepping over people's necks, then stepping over necks is permissible, because it is the fault of those people who, despite there being empty space, sat at the back. Similarly, the imam may also step over people's necks to reach the pulpit (minbar), because there is no alternative.

1118. Commentary:
➊ Arriving late for Jumu‘ah and then attempting to move forward by stepping over people’s necks is an extremely reprehensible act. Honoring a Muslim is obligatory, and causing him harm is forbidden.
➋ However, if due to ignorance people leave the front rows empty and sit at the back, then in such a case, stepping over their necks is permissible. This is because they themselves have forfeited their own honor by sitting at the back and not filling the front rows.
➌ Nevertheless, the khatib (preacher) or imam is granted concession for this act due to a legitimate Shar‘i need. Similarly, if someone loses his ablution (wudu), then it becomes necessary for him to go outside. Even then, he should pass through with respect and honor.