"הבל הבלים אמר קהלת " 1:2 – רש"י
"הבל הבלים הכל הבל" It says the word "הבל" three times and the word "הבלים" twice."הבלים" is a Loshon Rabbim, thereby each time it is stated counts as two, for a total of four - plus three times "הבל" – for a grand total of seven. The seven refers to the seven days of creation.