Delaying intention for optional fasts

The Messenger of Allah ﷺ said to me one day: "O Aishah, do you have anything?" She said: I said, "O Messenger of Allah, we have nothing." He said: "Then I am fasting." 

She said: The Messenger of Allah ﷺ went out, then a gift was given to us, or visitors came to us. She said: When the Messenger of Allah ﷺ returned, I said, "O Messenger of Allah, a gift was given to us, or visitors came to us, and I have kept something aside for you." He said: "What is it?" I said, "Hays." He said: "Bring it." So I brought it to him and he ate, then he said: "I had begun the morning fasting."

Talhah said: I narrated this hadith to Mujahid, and he said, "That is like a man who sets aside charity from his wealth: if he wishes, he may carry it through, and if he wishes, he may withhold it."

قَالَ لِي رَسُولُ اللَّهِ ﷺ ذَاتَ يَوْمٍ: «يَا عَائِشَةُ، هَلْ عِنْدَكُمْ شَىْءٌ؟» قَالَتْ: فَقُلْتُ: يَا رَسُولَ اللَّهِ، مَا عِنْدَنَا شَىْءٌ. قَالَ: «فَإِنِّي صَائِمٌ». قَالَتْ: فَخَرَجَ رَسُولُ اللَّهِ ﷺ، فَأُهْدِيَتْ لَنَا هَدِيَّةٌ أَوْ جَاءَنَا زَوْرٌ. قَالَتْ: فَلَمَّا رَجَعَ رَسُولُ اللَّهِ ﷺ قُلْتُ: يَا رَسُولَ اللَّهِ، أُهْدِيَتْ لَنَا هَدِيَّةٌ أَوْ جَاءَنَا زَوْرٌ، وَقَدْ خَبَأْتُ لَكَ شَيْئًا. قَالَ: «مَا هُوَ؟» قُلْتُ: حَيْسٌ. قَالَ: «هَاتِيهِ». فَجِئْتُ بِهِ فَأَكَلَ، ثُمَّ قَالَ: «قَدْ كُنْتُ أَصْبَحْتُ صَائِمًا».

قَالَ طَلْحَةُ: فَحَدَّثْتُ مُجَاهِدًا بِهَذَا الْحَدِيثِ، فَقَالَ: ذَاكَ بِمَنْزِلَةِ الرَّجُلِ يُخْرِجُ الصَّدَقَةَ مِنْ مَالِهِ، فَإِنْ شَاءَ أَمْضَاهَا، وَإِنْ شَاءَ أَمْسَكَهَا.