Musnad Ahmad 284:27

Jarir narrated to us, from Mansur, from Mujahid, from Yusuf ibn al-Zubayr, from Abdullah ibn al-Zubayr, who said: A man from Khath'am came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "My father has reached Islam while he is a very old man who cannot sit firmly on a mount, and the Hajj is prescribed for him. Shall I perform Hajj on his behalf?" He said: "Are you the eldest of his children?" He said: "Yes." He said: "Do you see, if your father had a debt and you paid it off on his behalf, would that suffice for him?" He said: "Yes." He said: "Then perform Hajj on his behalf."