The meaning of the saying of al-Sadiq, peace be upon him: «The plumb is the plumb of Hamran» and the meaning of «the plumbing line»

We gathered with Abu Abdillah, peace be upon him, among a group of his most glorious befrienders, and among us was Hamran bin A’yan. Then he went into a discussion, while Hamran was silent. Then Abu Abdillah, peace be…

Abu Abdillah, peace be upon him, said: Between you and between those who oppose you, there is nothing but the plumbing line. I said: And what thing is the plumbing line? He said: That which you call a plumb. Hence,…