﴿ وَأَصْبَحَ فُؤَادُ أُمِّ مُوسَىٰ فَٰرِغًا ۖ إِن كَادَتْ لَتُبْدِى بِهِۦ لَوْلَآ أَن رَّبَطْنَا عَلَىٰ قَلْبِهَا لِتَكُونَ مِنَ ٱلْمُؤْمِنِينَ ﴾

“On the morrow, however, an aching void grew up in the heart of the mother of Moses, and she would indeed have disclosed all about him had We not endowed her heart with enough strength to keep alive her faith [in Our promise].”

Please check ayah 28:13 for complete tafsir.