Musar zu Bereschit 21:6

He explained to Esau that through the power of the afflictions endured as a result of the attribute of Justice, G–d becomes able to employ the attribute of Mercy. So much on the mystical dimension of the three gifts the Jewish people receive only by means of יסורים. These experiences are already alluded to by Sarah when she exclaimed צחק עשה לי אלוקים, "G–d (in His capacity of the attribute of Justice) has made a joy for me (with the birth of יצחק)" (Genesis 21,6).