Praise be to Allah.
The fuqaha’ are agreed that abstaining from the things that break the fast, from the true dawn until sunset is one of the pillars of fasting.
But they differed with regard to the intention. The Hanafis and Hanbalis are of the view that the intention is a condition of the fast being valid.
The Maalikis and Shaafa’is were of the view that it is a pillar in addition to abstinence.
Whether the intention is regarded as a pillar or a condition, the fast – like other acts of worship – cannot be regarded as valid without the intention, as well as abstaining from the things that break the fast.
Al-Bahr al-Raa’iq, 2/276; Mawaahib al-Jaleel, 2/378; Nihaayat al-Muhtaaj, 3/149; Nayl al-Ma’aarib Sharh Daleel al-Taalib, 1/274.
And Allaah knows best.