Salim bin Abi al-Ja'd narrates from Jabir bin Abdullah (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to deliver the sermon standing on Friday. A caravan came from Syria, and the people went off towards it until only 12 men remained with him (peace and blessings be upon him). Then this verse was revealed: "And when they see a trade caravan or some amusement, they turn off towards it and leave you standing." [Surah al-Jumu'ah: 11]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5705