Musnad Ahmad 941:47

'Abd al-Razzaq narrated to us, Sufyan informed us, from 'Asim, from Abu 'Uthman al-Nahdi, from Usamah ibn Zayd, who said: The daughter of the Prophet (peace and blessings of Allah be upon him) sent [a message] to him: "My son is dying, so come to us." He sent [a messenger] to convey peace and say: "To Allah belongs what He took, and to Him belongs what He gave, and everything with Him has a specified term." She sent to him again, adjuring him to come. So he stood up, and we stood up with him: Mu'adh ibn Jabal, Ubayy ibn Ka'b, and Sa'd ibn 'Ubadah. He took the child, and his soul was rattling [in his chest]. His eyes shed tears. Sa'd said: "O Messenger of Allah, what is this?" He said: "This is a mercy that Allah has placed in the hearts of His servants, and Allah only shows mercy to the merciful among His servants."