Allah, the Almighty, ordered me to read to you {Those who disbelieve among the People of the Book and the Polytheists were not going to depart (from their ways) until there should come to them Clear Evidence.} [Sūrat al-Bayyinah: 98]
Authentic hadithعن أنس بن مالك رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم لأُبَي بنِ كعْب رضي الله عنه : «إن الله عز وجل أمَرَني أن أَقْرَأَ عَلَيك: (لم يكن الذين كفروا...) قال: وسمَّاني؟ قال: «نعم» فبكى أُبي. وفي رواية: فَجَعَل أُبَي يَبكِي.
Anas ibn Mālik (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said to Ubay ibn Ka‘b (may Allah be pleased with him): "Allah, the Almighty, ordered me to read to you {Those who disbelieve among the People of the Book and the Polytheists were not going to depart (from their ways) until there should come to them the Clear Evidence.} [Sūrat al-Bayyinah: 98]" Ubay asked: "Did He (Allah) name me?" The Messenger of Allah replied: 'Yes.' So Ubay cried. In another narration: "So Ubay started to cry."
Explanation
In this Hadīth, the Prophet (may Allah's peace and blessings be upon him) informs Ubay ibn Ka‘b that Allah ordered him to recite Sūrat al-Bayyinah to Ubay, so Ubay (may Allah be pleased with him) was amazed at how that could happen when the norm is that the less superior person reads to the one who is more superior, not vice versa. So when Ubay ascertained the matter from the Prophet (may Allah's peace and blessings be upon him) and received confirmation that Allah has mentioned his name, it made him cry out of joy and happiness that Allah, the Exalted, mentioned him by name.
Source · Narrated by Bukhari - Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).