The Book of Suckling
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ الْقَعْنَبِيُّ، حَدَّثَنَا سُلَيْمَانُ بْنُ بِلاَلٍ، عَنْ يَحْيَى، - وَهُوَ ابْنُ سَعِيدٍ - عَنْ عَمْرَةَ، أَنَّهَا سَمِعَتْ عَائِشَةَ، تَقُولُ - وَهْىَ تَذْكُرُ الَّذِي يُحَرِّمُ مِنَ الرَّضَاعَةِ - قَالَتْ عَمْرَةُ فَقَالَتْ عَائِشَةُ نَزَلَ فِي الْقُرْآنِ عَشْرُ رَضَعَاتٍ مَعْلُومَاتٍ ثُمَّ نَزَلَ أَيْضًا خَمْسٌ مَعْلُومَاتٌ .Amra reported that she heard 'A'isha (Allah he pleased with her) discussing fosterage which (makes marriage) unlawful; and she ('A'isha) said:There was revealed in the Holy Qur'an ten clear sucklings, and then five clear (sucklings)
From Sahih Muslim, whose reports the scholars accept as authentic.