Parashas Metzora · Passuk 14:21
Chabad.org — English translation + Rashi
Torah Temima points to Menachos 89a (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “For a minchah and a log of oil” — this is needed for one who voluntarily offers a minchah, that he not bring less than the amount that requires a log, which is an isaron.
Minimum for a free-will minchah
Chazal set the minimum quantity for a free-will minchah at one isaron, the amount that is accompanied by a log of oil.