Parashas Shemos
וְלֹא־יָכְלָ֣ה עוֹד֮ הַצְּפִינוֹ֒ וַתִּֽקַּֽח־לוֹ֙ תֵּ֣בַת גֹּ֔מֶא וַתַּחְמְרָ֥הֿ בַחֵמָ֖ר וּבַזָּ֑פֶת וַתָּ֤שֶׂם בָּהּ֙ אֶת־הַיֶּ֔לֶד וַתָּ֥שֶׂם בַּסּ֖וּף עַל־שְׂפַ֥ת הַיְאֹֽר׃
When she could hide him no longer, she got a wicker basket for him and caulked it with bitumen and pitch. She put the child into it and placed it among the reeds by the bank of the Nile.
Torah Temimah points to Sotah 12a (verified).
ולא יכלה עוד הצפינו — \ldots כל היכי דהוי שמעי מצראי דמתיליד ינוקא, ממטי ינוקא להתם\ldots
English: “And she could no longer hide him” — whenever the Egyptians heard that a child had been born, they would bring a [crying] infant there so that [the hidden child] would cry along with them.
Custom
The concealment became impossible once Egyptian search parties used this method.