Dmitri Hvorostovsky sings “Farewell Joy“ Russian Folksong

This is from the very end of a video I saw on this site: Barma (). Title is: “Хворостовский, Гаранча | Hvorostovsky, Garanca, Moscow “ Hvorostovsky just launches into this beautiful and bewitching folk song and does it acapella and seemingly spontaneously. Several people in the audience are in tears and so is Elīna Garanča. The conductor kisses Hvorostovsky at the end of it. Does anyone know more about this? Hadn’t he just recently returned to the stage after finding out about having cancer? He was such a light in the world -- and what a smile! Here are the lyrics: “Farewell, happiness, my life, I know you go around without me It means we must part; I won’t see you any more Dark little night! Ah, but I can not sleep! I myself don’t know why You, dear little girl, ... You alone disturb me, You alone deprived me of rest. (refrain; Dark etc) Remember, remember that May day, My dear one and I went to swim. We sat on the sand, On the yellow, the soft sand. Dark little night! Ah, but I can not sleep...“