Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "The acceptance of Islam by Umar (may Allah be pleased with him) was a victory, and his leadership was a mercy. By Allah, we were not able to pray at the House (the Ka'bah) until Umar (may Allah be pleased with him) accepted Islam. Then when Umar (may Allah be pleased with him) accepted Islam, he confronted the idolaters until they left us alone, and we prayed."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8788