Musnad Ahmad 20:19

Rawh narrated to us, Ibn Jurayj narrated to us, Ja'far ibn Khalid ibn Sarah informed me, that his father informed him, that 'Abdullah ibn Ja'far said: If you had seen me, Qutham, and 'Ubaydullah, the sons of 'Abbas, while we were young boys playing, when the Prophet, peace and blessings of Allah be upon him, passed by on a mount. He said: "Lift this one up to me." He said: So he carried me in front of him, and he said regarding Qutham: "Lift this one up to me," and he placed him behind him. 'Ubaydullah was more beloved to 'Abbas than Qutham, yet he did not feel shy before his uncle that he carried Qutham and left him. He said: Then he wiped my head three times, and each time he wiped it, he said: "O Allah, be the successor to Ja'far in his children." I said to 'Abdullah: What happened to Qutham? He said: He was martyred. I said: Allah knows best what is good, and His Messenger knows best what is good. He said: Yes.