Musnad Ahmad 28:68

Yazid narrated to us, Mis'ar informed us, from Habib ibn Abi Thabit, from Abu al-Abbas, from Abdullah ibn Amr, who said: A man came to the Prophet, peace and blessings of Allah be upon him, seeking his permission for Jihad. He said, "Are your parents alive?" He said, "Yes." He said, "Then in them, strive (perform Jihad)."