The intention to fast, and the differences reported from Talhah bin Yahya in the narration of 'Aishah about it
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا وَكِيعٌ، قَالَ حَدَّثَنَا طَلْحَةُ بْنُ يَحْيَى، عَنْ عَمَّتِهِ، عَائِشَةَ بِنْتِ طَلْحَةَ عَنْ عَائِشَةَ أُمِّ الْمُؤْمِنِينَ، قَالَتْ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم ذَاتَ يَوْمٍ فَقَالَ " هَلْ عِنْدَكُمْ شَىْءٌ " . قُلْنَا لاَ . قَالَ " فَإِنِّي صَائِمٌ " .
It was narrated that 'Aishah, the mother of the Believers, said: "The Messenger of Allah (ﷺ) came to me one day and said: 'Do you have anything (to eat) we said: 'No.' he said: 'Then I am fasting."