“One strip of white silk” is an ancient saying. It appears in various places, the most well known being Case 96 of The Book of Serenity:
Jiufeng lived at Shishuang’s monastery as his attendant. After Shishuang’s death, the gathering wanted the head student to succeed him as abbot. Jiufeng did not acknowledge him. He said, “Wait, let me examine him. If he understands what our late teacher meant, I will attend him as I did our late teacher.”
So he asked the head student, “Our late teacher said, ‘You should extinguish all delusive thoughts. You should let consciousness expire. You should let your one awareness continue 10,000 years. You should let your awareness become like winter ashes and a withered tree. You should let your consciousness become like one strip of white silk.’ Tell me, what was he trying to make clear by this?”
The head student said, “He intended to clarify the matter of absolute emptiness.”
Jiufeng said, “Then you do not yet understand our late teacher’s meaning.”
“You don’t acknowledge me? Pass me the incense.” He lit the incense and said, “If I don’t understand our late teacher’s meaning, I won’t be able to die while this incense is still burning.” No sooner had he said this than he sat down in meditation and died.
Jiufeng then caressed his shoulder and said, “Though you can die sitting or standing, you still have not dreamed of our late teacher’s meaning.”
—Book of Serenity, Case 96