"And she could no longer hide him" (Exodus 2:3). Why could she not simply keep hiding him and go on? Rather, whenever the Egyptians heard that an infant had been born, they would bring an infant of their own there and make it cry, so that the hidden child would hear and cry along with it, as it is written, "Catch us the foxes" (Song of Songs 2:15). "And she took for him an ark of bulrushes" (Exodus 2:3). Why specifically bulrushes? Rabbi Elazar said: From here we learn of the righteous, that their money is more precious to them than their bodies. And why so? Because they do not stretch out their hands to robbery, [so they used the cheapest material]. And Rabbi Shmuel bar Nahmani said: It is a soft material that can withstand both a soft encounter and a hard one. "And she daubed it with slime and with pitch" (Exodus 2:3) - the slime within and the pitch without, so that the righteous child should not smell the foul odor [of the pitch].
Why the Ark of Moses Was Built of Reeds and Sealed Within and Without
Yalkut Shimoni on Torah 166:2
(שמות ב ג) וְלֹא יָכְלָה עוֹד הַצְפִּינוֹ. אַמַאי תִּצְפְנִיה וְתֵּזִיל, אֶלָּא כָּל הֵיכָא דְשַׁמְעֵי מִצְרָאֵי דְמִתְיַלִיד יְנוּקָא, מַמְטֵי יְנוּקָא לְהָתָם וּמָעווּ כִּי הֵיכִי דְלִישְׁמְעוּהָ וּמָעוּי בַּהֲדֵיהּ, דִּכְתִיב (שה"ש ב, טו) "אֶחֱזוּ לָנוּ שׁוּעָלִים". וַתִּקַּח לוֹ תֵּבַת גֹּמֶא, מַאי שְׁנָא גֹּמֶא, אָמַר רַב אֶלְעָזָר, מִכָּאן לַצַדִּיקִים שֶׁמָּמוֹנָן חָבִיב עֲלֵיהֶן מִגּוּפָן, כָּל כָּךְ לָמָּה, שֶׁאֵין פּוֹשְׁטִים יְדֵיהֶם בְּגֶזֶל. וְרַבִּי שְׁמוּאֵל בַּר נַחְמָנִי אָמַר, דָּבָר רַךְ שֶׁיָּכוֹל לַעֲמֹד בִּפְנֵי רַךְ וּבִפְנֵי קָשֶׁה. וַתַּחְמְרָה בַחֵמָר וּבַזָּפֶת, חֵמָר מִבִּפְנִים, וְזָפֶת מִבַּחוּץ, כְּדֵי שֶׁלֹּא יָרִיחַ אוֹתוֹ צַדִּיק רֵיחַ רַע (כתוב ברמז נ"ג).