Sunni reference: Tafsir al-Kabir, by Fakhr al-Din al-Razi, v9 - Under
the explanation of the chapter of al-Kahf - 9 ]
Shi'i reference: Bihar al-Anwar, v40, pp 281-282

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On Imam Ali (AS) *
Kufa was now the seat of the Islamic rule. The entire Muslim world
with the exception of Damascus looked towards Kufa for guidance. They
travelled to Kufa often.
Two men were in each other's company on the road to Kufa. One was a
muslim and the other was a non muslim. As they progressed, the non muslim
traveller asked his companion:
I am going further than Kufa, to a small village where I live. Where
are You going?
His muslim companion said:
I live in Kufa
They proclaimed happily, discussing various things, helping each other, all
in a spirit of cordiality. When they drew near to Kufa, the non muslim took
a side road towards his village. Just as he was about to bid farewell, he
observed that his muslim companion was coming with him.
Didn't you tell me that You are going to Kufa where You lived?
The muslim replied
Yes of course
On this the non muslim inquired
Then why were you coming this way? That is the only road leading to
Kufa?
I know, said the muslim, But we have long been companions. Our Prophet
(PBUH&HF) said that when two are companions to each other on a journey
they have obligations and duties towards each other. You gave me
company and now it is my duty to follow You a few steps and then make
a gentle departure.
No wonder that Your Prophet (PBUH&HF) managed to spread his faith so
quickly. His (PBUH&HF) teachings were indeed great.
And then it so happened that once the same non muslim entered Kufa. There
he found that his muslim companion had been none other but the Khalifa of
his time, Ali ibn Abi Talib (AS). He soon became a muslim and remained the
most faithful of companion of the Imam.
To judge Your own souls before the time of judging of your
action arrives. Make an assessment of Yourself before You are called
upon to account for Your conduct in this existence.
[ Imam Ali (AS)