(a) Ibn Mas'ud (may Allah be pleased with him) said: "Do not form rows between the pillars."
(b) In the hadith of Abu Ishaq it is: "Do not form rows between the pillars."
Note: The pillars are an obstacle to joining the row. If a man is alone, or the row does not extend beyond the pillar, then there is no harm.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5206