Amr bin Shu'ayb narrates from his father, from his grandfather (Sayyiduna Abdullah bin Amr, may Allah be pleased with them both), that the Prophet (peace and blessings be upon him) forbade that a man sit between two men except with their permission.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5893