Sahih Bukhari
Fighting for the Cause of Allah (Jihaad)
Volume: 004 / Book Number: 052 / Hadith Number: 248
Narrated 'Abdullah bin 'Amr:
A man came to the Prophet asking his permission to take part in Jihad. The Prophet asked him, "Are your parents alive?" He replied in the affirmative. The Prophet said to him, "Then exert yourself in their service."