Hallel is a rabbinic commandment. As such there is no mention of it in the Torah (Pentateuch). There is also no (explicit) mention in the rest of Tanach.
What we do have is the Talmud in Arachin 10a which tells us on which days we are obligated to recite Hallel.
The Talmud in Shabbat 23a asks (with reference to the similarly rabbinic commandment to light Chanukah lights) how we can say in the blessing that G-d commanded us to perform this commandment, when it is rabbinic in nature?
Two answers are given:
רב אויא אמר מלא תסור רב נחמיה אמר שאל אביך ויגדך זקניך ויאמרו לך
Rav Avya said, “From ‘You shall not depart’ (Devarim 17:11).” Rav Nehemiah said, “‘Ask your father and he will tell you, your elders and they will say to you’ (Devarim 32:7).”