In Romeo and Juliet, what quote best proves that Friar Lawrence came up with the plan to fake Juliet's death?

While my earlier answer details the entire plan, your question asked for one specific quote.  If I were to choose only one, it would be lines 96-101 in Act 4, Scene 1:

Take thou this vial, being then in bed,                                            And this distillèd liquor drink thou off,                                           When presently through all thy veins shall run                                    A cold and drowsy humor, for no pulse                                             Shall keep his native progress, but surcease.                                No warmth, no breath shall testify thou livest.

Here, the Friar lays out the plan that Juliet will drink a concoction that will make her appear dead.  This portion of text later goes on to say that she will awaken two days later fully functional and within the tomb of her family.  At that point, she will be able to escape with Romeo without the knowledge of her family.