Sayyiduna Abu Juhayfah (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Whoever fills an empty space in the row is forgiven."
Al-Bazzar recorded it, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2503