Wa `Alaykum As-Salamu Wa-ramatullah Wa-barakatuh

The prophet (peace be upon him) said: “Make your rows straight, stand shoulder to shoulder and close the gaps, and do not resist your brothers’ hands. Do not leave any gaps for the Shaytan. Whoever completes a row, Allah will reward him, and whoever breaks row, Allah will forsake him.”

As you see, the shoulders are the main cause to straighten rows and keep Muslims close to each other because each person opens wide space different between his feet from the other.

Although there are some narrations about how the Sahabah used to have foot next to each other but the exact words of the prophet (peace be upon him) as mentioned above specify the shoulders. And by default, if shoulders are close to each other, the feet will be automatically.

Almighty Allah knows best.