The verse declaring the perfection and completion of the Islamic religion appears in Surah Al-Ma'idah (5:3). The relevant portion reads: "This day I have perfected for you your religion and completed My favour upon you and have approved for you Islam as religion."
This verse was revealed during the Prophet's Farewell Pilgrimage at the plain of Arafat in 10 AH, and it is one of the most significant verses in the Qur'an because it signals that the legislative framework of Islam was now complete.
The other surahs, while containing important legal and theological content, do not contain this declaration. Al-Baqarah (Surah 2) covers many foundational rulings. Al-Imran (Surah 3) deals extensively with the people of the scripture and the Battle of Uhud. Al-Nisa (Surah 4) focuses on family law, inheritance, and women's rights. The perfection of the religion, however, is specifically stated in Al-Ma'idah.