Hallelujah, praise the servants of the Lord. This is what the verse (Psalm 137:7) means when it says, "I will remember my song in the night." What is my song? Rabbi Yivo said that the congregation of Israel says before the Holy One, Blessed be He, "I remember Your covenant."
As it says (Genesis 14:20), "Blessed be God Most High, Who has delivered your enemies into your hand." Therefore, "with all my heart I will sing praises to You." Rabbi Yehuda, son of Rabbi Simon, said that the congregation of Israel says, "I remember the miracles that You did for me at night in Egypt, and I sang to You and recited songs and praises at night." As it says (Isaiah 30:29), "You shall have a song as in the night when a holy feast is kept."
When did this happen? When You struck down the firstborn of Egypt at night, as it says (Exodus 12:29), "And it came to pass at midnight." This is when we remember our song. On that same night we were redeemed and went out of slavery to freedom.
We were slaves to Pharaoh, but You redeemed us and made us Your servants, as it says, "Hallelujah, praise the servants of the Lord."