Abd al-Wahhab bin Ata (may Allah have mercy on him) says that Sa'id bin Abi Arubah (may Allah have mercy on him) was asked about the virtue of the first row, and he narrated from Qatadah (may Allah have mercy on him), from Sayyiduna Anas bin Malik (may Allah be pleased with him), that the Messenger of Allah (peace and blessings be upon him) said: "Complete the first row, then the one next to it; and whatever shortfall there is, let it be in the last row."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5191