Commentary for Berakhot 31:4

קרא וטעה ואינו יודע להיכן טעה באמצע הפרק יחזור לראש בין פרק לפרק יחזור לפרק ראשון בין כתיבה לכתיבה יחזור לכתיבה ראשונה

Should one have read the Shema' and erred without knowing for certain where he made the mistake, if this occurred in the middle of the first paragraph he must return to the commencement ; if between two paragraphs, he must return to the first of them ; if between "and thou shalt write them" and "ye shall write them," he must return to the first.

Abraham Cohen Footnotes to the English Translation of Masechet Berakhot