I heard that you shouldn't count your money because of the ayin hara (Evil Eye) or something like that. Can anybody bring the source for it?
On the verse (Ex. 30:12)
כִּ֣י תִשָּׂ֞א אֶת־רֹ֥אשׁ בְּנֵֽי־יִשְׂרָאֵ֘ל לִפְקֻדֵיהֶם֒ וְנָ֨תְנ֜וּ אִ֣ישׁ כֹּ֧פֶר נַפְשׁ֛וֹ לַיהוָ֖ה בִּפְקֹ֣ד אֹתָ֑ם וְלֹא־יִהְיֶ֥ה בָהֶ֛ם נֶ֖גֶף בִּפְקֹ֥ד אֹתָֽם׃
Rashi cites the sages who commented (Sefaria trans.):
THAT THERE BE NO CALAMITY AMONG THEM — for numbers (i. e. things that have been numbered) are subject to the influence of the “evil eye”, and therefore if you count them by their polls pestilence may befall them,
The BT (BM 42a) comments:
וא"ר יצחק אין הברכה מצוייה אלא בדבר הסמוי מן העין שנאמר (דברים כח, ח) יצו ה' אתך את הברכה באסמיך תנא דבי רבי ישמעאל אין הברכה מצויה אלא בדבר שאין העין שולטת בו שנאמר יצו ה' אתך את הברכה באסמיך
And Rabbi Yitzḥak says: Blessing is found only in a matter concealed from the eye, as it is stated: “The Lord will command blessing with you in your storehouses” (Deuteronomy 28:8), where the grain is concealed. The school of Rabbi Yishmael taught: Blessing is found only in a matter over which the eye has no dominion, as it is stated: “The Lord will command blessing with you in your storehouses.
The Gemoro Bovo Metzia 42a has a passage that is reminiscent of your question.
R. Isaac also said: A blessing is found only in what is hidden from the eye,5 for it is written, "The Lord shall command the blessing upon thee in thy hidden things."6
The School of R. Ishmael taught: A blessing comes only to that over which the eye has no power,7 for it is said, “The Lord shall command the blessing upon thee in thy hidden things.”
Our Rabbis taught: When one goes to measure [the corn in] his granary, he should pray, 'May it be Thy will, O Lord our God, to send a blessing upon the work of our hands.' Having started to measure, he prays, 'Blessed is He who sends a blessing on this pile.'
But if he measured and then prayed, it is a vain prayer, because a blessing is not found in that which is [already] weighed, measured, or counted, but only in that which is hidden from the eye, for it is said, “The Lord shall command the blessing upon thee in thy hidden things.”
i.e., the exact quantity of which the owner does not know.
Deuteronomy XXVIII, 8.
I.e., hidden, and so not subject to the evil eye.