The stirring song is sung on several occasions in the Jewish calendar most prominently on Shavuos and Simchas Torah there is great debate on who composed the niggun and where the lyrics come from some attribute it to the Chazon Ish, Rav Meir Shapira, others say the vilna gaon, click HERE for more. Article in Mishpacha Magazine HERE the story behind the song. Watch this video below for a different niggun sung by Benny Frieman to the words of Kad Yasvun. כד יתבין ישראל ועסקין בשמחת התורה, קודשא בריך הוא אומר לפמליא דיליה חזו בני חביבי דמשכחין בצערא דילהון ועסקין בחדוותא דילי
English translation: When Klal Yisroel are sitting and engaging in Torah study, the Holy One, blessed is He, says to his heavenly army: 'See! See! My beloved children who forget about their personal problems and engage in My delight'."
source: Thepartialview