The daughter of the Prophet ﷺ sent word to him, "A son of mine has died, so come to us."He sent back greetings of peace and said, "Indeed, to Allah belongs what He takes and to Him belongs what He gives, and everything with Him has an appointed term. So let her be patient and seek reward."She then sent word to him again, beseeching him to come to her. So he set out, and with him were Saad bin Ubada, Muaz bin Jabal, Ubayy bin Kaab, Zayd bin Thabit, and some men. The child was lifted up to the Prophet ﷺ with his soul rattling in his chest. I think he said, "like a worn-out waterskin." The Prophet's ﷺ eyes overflowed with tears.Saad asked, "Messenger of Allah, what is this?"He replied, "This is mercy that Allah has placed in the hearts of His servants. Allah shows mercy only to those of His servants who are merciful."
أرسلت ابنة النبي ﷺ إليه إن ابنا لي قبض، فأتنا، فأرسل يقرئ السلام، ويقول: إن الله ما أخذ، وله ما أعطى، وكل عنده بأجل مسمى، فلتصبر ولتحتسب فأرسلت إليه تقسم عليه ليأتينها، فقام ومعه سعد بن عبادة، ومعاذ بن جبل، وأبي بن كعب، وزيد بن ثابت ورجال، فرفع إلى النبي ﷺ صبي، ونفسه تتقعقع، قال: حسبته أنه قال: كأنها شن، ففاضت عيناه فقال سعد: يا رسول الله ما هذا؟ فقال: هذه رحمة جعلها الله في قلوب عباده، وإنما يرحم الله من عباده الرحماء.
It was narrated by Bukhari in Funerals (1284) and Muslim in Funerals (923), both through Asim bin Sulayman Ahwal, from Abu Uthman Nahdi, from Usama bin Zayd, who related it. The wording given is Bukhari's, and Muslim's wording is close to it.After returning to the books of history and biography, it became clear to me that the daughter of the Prophet ﷺ who sent for him was Fatimah, and the little boy was Muhsin bin Ali bin Abu Talib, since the scholars of historical reports agree that he died young during the lifetime of the Prophet ﷺ.It has also been said that the daughter of the Prophet ﷺ who sent for him was Zaynab, but this raises a difficulty regarding the identity of the boy who died, since Zaynab bore no child except Ali bin Abu As bin Rabi, who had nearly reached puberty on the day of the conquest, and Umamah, who lived on after the Prophet ﷺ and whom Ali bin Abu Talib married after the death of Fatimah.