Not just “I saw it happen.” A stake: putting one’s word behind what happened, and being held to it.
English “witness” mostly means passive perception — someone who happened to be present when something occurred. The Arabic root behind shahada (testimony, and the declaration of faith) and shahid carries a thicker sense: to witness is to testify, publicly and accountably, not merely to have observed privately. Bearing witness is an act with consequences for the witness, not a neutral report filed from a safe distance.
The same root produces shahid, ordinarily translated “martyr” in English but literally “witness” — someone whose testimony to a truth is understood as complete, borne out with everything they had rather than only with their word. Legal-historical scholarship traces how this originally testimonial meaning — bearing witness to truth, at cost to oneself if necessary — later became entangled with specific and contested claims about violent death in war, a development scholars treat as a later reinterpretation layered onto, not identical with, the older and broader sense of accountable testimony.
Two people who trust each other as witnesses, in that older sense, are staking their word on each other’s account being true — a smaller-scale version of the same structure: testimony that carries weight because the witness can be held to it.
Source: Freamon, B.K., “Martyrdom, Suicide, and the Islamic Law of War: A Short Legal History,” Fordham International Law Journal, 2003; Cook, D., “Martyrdom (Shahada),” Oxford Bibliographies in Islamic Studies, 2012.