Sahih Al Bukhari Hadith Number 723

Narrated Anas bin Malik: The Prophet (peace be upon him) said, “Straighten your rows as the straightening of rows is essential for a perfect and correct prayer. “

10  Call to Prayers (Adhaan)
Hadith number 723
Volume no 1
Chapter: 74 (The straightening of the rows is amongst those obligatory and good things which make your As-Salat (the prayer) a correct and perfect one.)