Sayyiduna Jabir bin Samurah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) came out to the rows and said: "Will you not form rows as the angels form rows before their Lord?" The Companions (may Allah be pleased with them) asked: How do the angels form rows before Allah? He (peace and blessings be upon him) said: "They first complete the front rows, and they close up the rows."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5189
Sayyiduna Anas (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Complete the front row first, then the second row; and if there is any shortfall, let it be in the last row."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5190
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