Muqatil bin Hayyan (may Allah have mercy on him) says that the Prophet (peace and blessings be upon him) said: "If a man comes and does not find anyone, let him pull a man from the row to himself so that he stands with him; how great is the reward of the one who is pulled!"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5212