The text of the Torah refers to the sea "splitting" (וַיִּבָּקְעוּ הַמָּיִם - Ex.14:21). Where is the first source where the Sages refer to this event as "tearing" (קריעה)?
It shows up in תוספתא ברכות ב:א. I cannot find an earlier source.
I think that if Dr. Ernest Goldschmidt's hypopthesis regarding the דיינו poem is correct - that it was composed during the glory days of the temple, seeing as it's the poem's concluding theme - then it might be the oldest source.
A thought of mine on the origins of the phrase: maybe it's based on the Pasuk in Tehillim 136: "לגוזר ים סוף לגזרים", but since "גזירת ים סוף" would have a different meaning in Mishnaic Hebrew they used the term "קריעה".