Musannaf Ibn Abi Shayba 37:181

Abu Mu'awiyah narrated to us, from al-A'mash, from Shaqiq, from Hudhayfah, who said: We were with the Prophet, may Allah bless him and grant him peace, and he said: "Count for me everyone who has uttered Islam." He said: We said, "O Messenger of Allah, do you fear for us while we are between six hundred and seven hundred?" He said: "You do not know; perhaps you will be tested." He said: So we were tested, until a man among us would not pray except in secret.