It is narrated from Sayyiduna Bara’ bin Azib (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Keep your rows straight so that there may not be discord in your hearts.” The narrator said: And he (peace be upon him) used to say: “Indeed, Allah sends mercy and His angels pray for those in the first row,” or he said: “for those in the first rows.”
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1298
(Commentary on Hadith 1297) That is, completing the row (saff) is a cause of reward. In one hadith it is stated: Whoever joins the row, Allah Ta'ala will join him with His mercy, and whoever cuts the row, Allah Ta'ala will cut him off from His mercy. See: [أبوداؤد 661], [نسائي 822]. In this hadith, there is the virtue of those who stand in the first row.