Musnad Ahmad 941:34

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Asim al-Ahwal, he said: I heard Abu 'Uthman narrating from Usamah ibn Zayd, he said: One of the daughters of the Messenger of Allah (ﷺ) sent to him [saying] that a child of hers—a son or a daughter—was dying, so witness us [attend]. He said: So he sent to her, greeting her with peace and saying: "Indeed, to Allah belongs what He took and what He gave, and everything with Him has a specified term. So let her be patient and seek reward." She sent [a messenger] swearing upon him [to come]. So he stood up, and we stood up. The child was lifted into the lap, or into the lap of the Messenger of Allah (ﷺ), and his soul was rattling. Among the people were Sa'd ibn 'Ubadah and Ubayy, I think. Then the eyes of the Messenger of Allah (ﷺ) overflowed with tears. Sa'd said to him: "What is this, O Messenger of Allah?" He said: "This is a mercy which Allah places in the hearts of whom He wills of His servants, and Allah only shows mercy to the merciful among His servants."