Allah SWT says:


يٰٓأَيُّهَا الَّذِينَ ءَامَنُوٓا إِذَا نُودِىَ لِلصَّلٰوةِ مِنْ يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلٰى ذِكْرِ اللَّهِ وَذَرُوا الْبَيْعَ ۚ ذٰلِكُمْ خَيْرٌ لَّكُمْ إِنْ كُنْتُمْ تَعْلَمُونَ

"O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew."
(QS. Al-Jumu'a 62: Verse 9)

* Via the Beautiful Qur'an http://quran-en.com