Sayyiduna Ibn Abbas (may Allah be pleased with them both) says, regarding the saying of Allah, the Mighty and Majestic: "And when they hear what has been sent down to the Messenger, you see their eyes overflowing with tears" [al-Ma'idah: 83]: These were sailors who had come with Sayyiduna Ja'far bin Abi Talib (may Allah be pleased with him) from Abyssinia. When the Messenger of Allah (peace and blessings be upon him) recited the Qur'an to them, they believed and their eyes overflowed. The Messenger of Allah (peace and blessings be upon him) said: "Perhaps when you return to your land you will turn away from your religion." They said: We will never turn away from our religion. So Allah, the Mighty and Majestic, revealed (this verse) on account of that saying of theirs.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12402