Reference for Berakhot 31:5

אמר ליה ר' יוחנן לא שנו אלא שלא פתח בלמען ירבו ימיכם אבל פתח בלמען ירבו ימיכם סרכיה נקט ואתי

R. Johanan said to him : This latter only applies if he has not started "That your days may be multiplied" ; but should he have commenced that sentence, [he may assume that] he followed on from force of habit.

Jastrow