TgJ Jer14:12 :אֲרֵי יְצוּמוּן לָא רַעֲוָא קְֹדָמַי לְקַבָלָא צְלוֹתְהוֹן‏ : for when they fast I am not willing to accept their prayer
See the entire entry for ṣwm vb. to fast