This is a reference to a Sunnī source on the unlawfulness of assassination. Hossein Modarressi draws on this source to show various Muslim legal views (including that of Jaʿfar al-Ṣādiq and the legal legacy of the House of the Prophet) on the impermissibility of assassination.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).