When it is said
Isaac gave it to his readers to say whenever they came to the Scriptures, before they began to read.
It is one line, so it fits before any reading, whether a whole chapter or a single verse.
Where it comes from
Author known
Who was Isaac the Syrian?
Isaac was a monk who was made bishop of Nineveh, in today’s Iraq, about AD 670. In the Orthodox Church in America’s life of him, two men brought him a quarrel over a debt. Isaac quoted the Gospel and asked the lender to give the debtor more time. The lender told him to leave his Gospel out of it. Isaac asked what was left for him to do there, and after five months as bishop he went to live with the hermits.
Encyclopaedia Britannica, “Isaac of Nineveh”; The Orthodox Church in America, the life of St Isaac the Syrian
Where the line comes from
Isaac wrote his homilies in Syriac. They were put into Greek at an early date, and they shaped Byzantine and Russian writers for centuries. In a homily made of short pieces of advice, he warns the reader never to come to the mysterious words of the Scriptures without praying for God’s help, and he gives this line to say. To Isaac, prayer is the key that opens the truth of Scripture.
Isaac of Nineveh, Mystic Treatises, translated by A. J. Wensinck (1923), Treatise 45, page 220; The Russian translation from the Greek, Homily 85 (Azbyka.ru); Encyclopaedia Britannica, “Isaac of Nineveh”