Assalaamu alaykum. Is it obligatory upon the imam to command those praying behind him to make the row perfect and straight and to fill the gaps before starting the prayer? And if those praying behind him do not make the row complete, will the imam be sinful?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

It is recommended – not obligatory (i.e. not Waajib) – for the imaam to command those praying behind him to straighten the rows, following the example of the Prophet, sallallahu ‘alayhi wa sallam. Imaam An-Nawawi said:

"It is recommended for the imaam to command those praying behind him to straighten the rows, based on the narration of Anas, may Allah be pleased with him, who related that the Messenger of Allah, sallallahu ‘alayhi wa sallam, said, 'Stand straight in your rows and fill the gaps, for I can see you from behind my back.' Anas added: 'Every one of us would stand shoulder to shoulder with his companion and foot to foot with him...' It is an act of the Sunnah for the imaam to command the people praying behind him to straighten the rows right before he recites the opening Takbeer (saying: Allahu Akbar [Allah is Most Great]). If the mosque is big, it is recommended for him to delegate a man to command them to straighten the rows and fill the gaps while passing around the rows and calling out to them to make the rows straight. It is also recommended for each of them to command others to straighten the rows if he notices that the rows are not straightened and the gaps are not filled. This falls under the category of enjoining good and cooperating in righteousness and piety. Straightening the rows means completing the first row and then the one behind it and so on, while also filling the gaps. The worshippers should stand next to each other so that no one's chest or any other part of his body is protruding from the row. They should not start a new row until the one before it is complete, and so on." [Al-Majmooʻ]

Ibn Rajab cited the following hadeeth in his book Fat-h Al-Baari: 

Anas, may Allah be pleased with him, said, "The Iqaamah (second, small call for prayer) was proclaimed. The Messenger of Allah turned to face us and said, 'Straighten your rows and stand next to each other, for I can see you from behind my back.'" Ibn Rajab commented, "This indicates that it is recommended for the imaam to face those led in prayer after the Iqaamah is proclaimed and command them to straighten their rows."

The imaam will not be sinful if those praying behind him did not straighten their rows. Straightening the rows is recommended and not obligatory according to the majority of scholars. Please refer to fatwas 84568 and 322945.

Allah knows best.