Hadith: It was revealed in the Qur'an that: ten times of breastfeeding known with certainty establish prohibition (Mahram relationship through breastfeeding); then that was abrogated and replaced with five times of breastfeeding known with certainty. The Messenger of Allah (may Allah's peace and blessings be upon him) died while these (words of the abrogating verse) were still being recited as part of the Qur'an

عن عائشة، أنها قالت: "كان فِيمَا أُنْزِلُ من القرآن: (عَشْرُ رَضَعَاتٍ مَعْلُومَاتٍ يُحَرِّمْنَ)، ثم نُسِخْنَ بِخَمْسٍ مَعْلُومَاتٍ، فَتُوُفِّيَ رَسُولُ الله صلى الله عليه وسلم ، وهُنَّ فِيمَا يُقْرَأُ من القرآن".
[صحيح] - [رواه مسلم]

‘Ā'ishah (may Allah be pleased with her) reported: It was revealed in the Qur'an that: ten times of breastfeeding known with certainty establish prohibition (Mahram relationship through breastfeeding); then that was abrogated and replaced with five times of breastfeeding known with certainty. The Messenger of Allah (may Allah's peace and blessings be upon him) died while these (words of the abrogating verse) were still being recited as part of the Qur'an.
[Authentic hadith] - [Narrated by Muslim]

Explanation

According to this Hadīth, the Qur’an initially stated that ten times of confirmed breastfeeding make the breastfed person Mahram (unmarriageable) to those who shared breastfeeding from the same woman and their Mahram relatives. Yet, both the ruling and wording of this verse were abrogated and replaced with another verse stipulating five times of confirmed breastfeeding. Later, only the wording of the latter verse was abrogated while its ruling remained in force. As for ‘Ā'ishah's statement that the Prophet died while this verse was still being recited as part of the Qur'an, it means that the abrogation came so late that news of it did not reach some people and, thus, they continued reciting the abrogated verse as part of the Qur’an due to their ignorance of its abrogation.