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wa-ʾidhā samiʿū mā ʾunzila ʾilā r-rasūli tarā ʾaʿyunahum tafīḍu mina d-damʿi mimmā ʿarafū mina l-ḥaqqi yaqūlūna rabbanā ʾāmannā fa-ktubnā maʿa sh-shāhidīna
When they hear what has been revealed to the Apostle, you see their eyes fill with tears because of the truth that they recognize. They say, ‘Our Lord, we believe; so write us down among the witnesses.
— Ali Quli Qarai
Surah Al-Ma'idah, Verse 83
83. "And when they hear what has been sent down to the Messenger, you see their eyes overflowing with tears because of what they recognize of the Truth. They say: 'Our Lord! we believe, so write us down with the witnesses (of Truth)."
One of the examples of the Christians' shedding tears is when Ja'far-ibn-'Abi-Talib recited the verses of Surah Maryam for Najashi in Abyssinia. Another example is also when a group of the Christians, accompanying with Ja'far, came into Medina and heard the holy verses of Surah yasin.
Therefore, as soon as the heartily receptive persons hear the Truth, they will be touched at heart, but the blockhead ones do not move even by seeing it. The spirit and nature of man is charmed by the Truth. Then, when he reaches his beloved, he sheds the tears of joy. However, believers should offer a petition for the continuation of their belief and having a good end.