Sahih Bukhari, Hadith: Q725, (76) Chapter. To stand shoulder to shoulder and foot to foot in the row.

صحيح البخاري

Sahih Bukhari

كتاب الأذان

The Book of Adhan

باب إلزاق المنكب بالمنكب والقدم بالقدم فى الصف:

(76) Chapter. To stand shoulder to shoulder and foot to foot in the row.