A man was seen with Umar ibn al Khataab and he said to him: “I don’t know you, and it does not harm you if I don’t know you, go get someone who knows you.

Then someone  said: ‘I know him’, so Umar replied: ‘what do you know about him? The man replied: ‘I know him to be a man of righteousness and virtue.

Umar said: ‘Is he your neighbor about whom you know what he does at night and during the day, and do you know when he leaves his house and comes home?

The man replied: “No”

Umar said to him: “Have you ever dealt with him in any business transactions, which prove one’s piety”

The man replied: “No”

Umar then asked him: ‘Has he ever accompanied you on a journey, from which you will know his manners?

The man replied: “No”

Umar then replied: “ You do not know this man”

Then Umar said to the man: “Go and bring someone who knows you.” End of quote.

Reference: Fiqh as Sunnah, Sayed Sabiq, Dar Taybatu al Khadrah, vol. 3, pg. 231