Bereshit 7:20

חמש עשרה אמה מלמעלה FIFTEEN CUBITS ABOVE —above the summits of all the mountains when once the waters reached the level of the mountains (Yoma 76a).

חֲמֵ֨שׁ עֶשְׂרֵ֤ה אַמָּה֙ מִלְמַ֔עְלָה גָּבְר֖וּ הַמָּ֑יִם וַיְכֻסּ֖וּ הֶהָרִֽים׃

Fifteen cubits upward did the waters prevail; and the mountains were covered.