Chasidut zu Tehillim 71:78

There are times when you desire to pray with passion but cannot, because your limbs feel heavy to you. The Baal Shem Tov of blessed memory said that King David prayed on this matter, saying (Psalms 71:9) "Do not cast me away in my old age" - meaning, when my limbs are heavy as if I were old.

The Baal Shem Tov explained the verse (Psalms 71:9) "Do not cast me away in the time (le'et) of my old age." This means that you should not cast away the moment (et) and let it grow old. For sometimes it happens that your worship grows old as time goes by.

The words of the divine master, the Baal Shem Tov of blessed memory. Regarding the verse (Psalms 71:9) "Do not cast me away in my old age," one could ask - did he not need help also in his youth? Rather, the meaning is that sometimes you have passion for something, Torah or prayer, but this desire doesn't mature but rather drops off suddenly. It is as if the mitzvah grew old, as in the phrase (cited in Tur, Orach Chaim 61) "like an old letter." It was in this regard that he said: Don't cast me away from passion! Don't let me see myself as an old person reading an old letter - let it become a new letter for me!