Musannaf Ibn Abi Shayba 27:339

'Abdullah bin Numayr narrated to us, from 'Abdul-Malik bin Abi Sulayman, from Wasil al-Ahdab, from Zayd bin Wahb, from 'Abdullah, who said: 'Umar was a fortified stronghold for Islam; people would enter Islam and not exit from it. When 'Umar was killed, the stronghold was breached; so people are exiting from Islam and not entering into it.