Yahya ibn Subayh said: Ammar client of al-Harith ibn Nawfal told me that he attended the funeral of Umm Kulthum, and her son. The body of the boy was placed near the imam. I objected to it. Among the people there were Ibn Abbas, AbuSa'id al-Khudri, AbuQatadah and AbuHurayrah. They said: This is the sunnah (established practice of the Prophet).
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَزِيدُ بْنُ خَالِدِ بْنِ مَوْهَبٍ الرَّمْلِيُّ، حَدَّثَنَا ابْنُ وَهْبٍ، عَنِ ابْنِ جُرَيْجٍ، عَنْ يَحْيَى بْنِ صُبَيْحٍ، حَدَّثَنِي عَمَّارٌ، مَوْلَى الْحَارِثِ بْنِ نَوْفَلٍ أَنَّهُ شَهِدَ جَنَازَةَ أُمِّ كُلْثُومٍ وَابْنِهَا فَجُعِلَ الْغُلاَمُ مِمَّا يَلِي الإِمَامَ فَأَنْكَرْتُ ذَلِكَ وَفِي الْقَوْمِ ابْنُ عَبَّاسٍ وَأَبُو سَعِيدٍ الْخُدْرِيُّ وَأَبُو قَتَادَةَ وَأَبُو هُرَيْرَةَ فَقَالُوا هَذِهِ السُّنَّةُ .
sunnah.com