«فِي أَصْلِ السَّنَدِ، أَيْ فِي الْمَوْضِعِ الَّذِي يَدُورُ الْإِسْنَادُ عَلَيْهِ وَيَرْجِعُ»
The origin of the chain is the pivotal point to which the various transmission branches ultimately return.
The technical idea of a common pivot matters here. Imagine twenty later students transmitting a report, but all twenty trace back to one person who was alone in transmitting it from his own teacher. The growth of later branches does not eliminate that earlier singularity.
Two distinct questions must be answered: how many later channels carry this report, and where do those channels converge upon a single earlier route? That inquiry is central to distinguishing absolute from relative singularity.