It is narrated from Hazrat Nu'man (may Allah be pleased with him) that once he came to lead us in prayer and saw a man whose chest was protruding out of the row. The Noble Prophet (peace and blessings be upon him) said: "Straighten yourselves, do not move forward or backward, otherwise differences will arise in your hearts as well."
Hadith Referenceمسند احمد / أول مسند الكوفيين / 18435
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن