Abd al-Hamid bin Mahmud says: We were with Sayyiduna Anas bin Malik (may Allah be pleased with him) in the row, and they pushed us between the pillars, so he moved back. When he had prayed, he said: We used to avoid this in the time of the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5204
Mu'awiyah bin Qurrah (may Allah have mercy on him) narrates from his father (may Allah be pleased with him) that in the time of the Prophet (peace and blessings be upon him) we used to be prevented from "standing between the pillars in prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5205
(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