From Sahih Bukhari (Hadith 2064):
Narrated Jabir:A caravan arrived (at Medina) while we were offering the Jumua prayer with the Prophet. The people left out for the caravan, with the exception of twelve persons. Then this Verse was revealed: 'But when they see some bargain or some amusement, they disperse headlong to it and leave you standing
Reference: Bukhari 2064