Reference for Keritot 36:4

אי אכל ראשון שומן הוא אכל חלב ליהוי כפרה ואי לא ליהוי נדבה

Our Rabbis taught: If one ate doubtful heleb and came to know of it,<span class="x" onmousemove="('comment',' At the time of eating he assumed it was permitted fat, but later learnt that there was some doubt about it.');">³</span>

Jastrow