Is it necessary to repeat the Dhuhr prayer according to the Shafi'i school if one prayed Jumu'ah behind a Hanafi Imam, and the number of worshippers was less than forty, or is it sufficient to rely on the Imam's belief?

The Shafi‘is and Hanbalis held that the Friday prayer is not valid with fewer than forty men. The Hanafis, and a narration from Imam Ahmad chosen by Shaykh al-Islam Ibn Taymiyyah, are of the view that it is valid with fewer than that. The Hanafis stipulate three or more along with the imam, while Abu Yusuf stipulates two besides the imam.

If the imam believes that a specific number of worshippers is required for the validity of the Friday prayer, and the number falls short of that, then it is not permissible for him to lead the worshippers. He is obliged to appoint someone else to lead them in prayer. Similarly, if the worshippers believe that a specific number is required and it is not met, then they should pray it as Dhuhr (noon prayer).