How sound is the statement attributed to Al-Hasan (ibn Ali) that he said regarding masturbation, "We used to do it during campaigns"?

It was narrated from Al-Hasan Al-Basri that masturbation is permissible, while disliking it was narrated from 'Ata', and absolute permissibility from Al-Hasan, 'Amr ibn Dinar, Ziyad Abi Al-'Ala', and Mujahid. The verifying scholars interpreted what was narrated from them as applying to necessity in military campaigns and the like, and not as an absolute permissibility.