Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Indeed, the acceptance of Islam by Umar (may Allah be pleased with him) was a victory, his emigration was a help, and his leadership was a mercy. By Allah, we were not able to pray openly at the Ka'bah until Umar (may Allah be pleased with him) accepted Islam."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8774