The Prophet was asked about whether Umrah was obligatory? He said: "No. But if you perform Umrah it is more virtuous."  

أَنَّ النَّبِيَّ ﷺ سُئِلَ عَنِ الْعُمْرَةِ أَوَاجِبَةٌ هِيَ قَالَ لاَ وَأَنْ تَعْتَمِرُوا هُوَ أَفْضَلُ