[Ruling on one who holds a deposit which was lost or stolen]

Answer—and Allah grants success: If the deposit was stolen from a place in which such a deposit is customarily kept for safekeeping, it is not guaranteed (no liability). If it was stolen from a place not customarily used for safekeeping, then it is guaranteed. If he forgot it somewhere until it was stolen, it is likewise guaranteed.
As for if it was simply lost, it is guaranteed.
In sum: If there was negligence in safeguarding on the part of the custodian, it is guaranteed; if there was no negligence, there is no liability. The jurists of the madhhab deemed loss and forgetfulness to be negligence.
The evidence is what is known in the Sacred Law of the obligation to safeguard trusts and then render them to their owners.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2