Talmud Bavli –Sukkah 52a:3

Granted, according to the one who said that the lament is for Messiah ben Yosef who was killed, this would be the meaning of that which is written in that context: “And they shall look unto Me because they have thrust him through; and they shall mourn for him, as one mourns for his only son” (Zechariah 12:10). Talmud Bavli, –Sukkah 52a:3