بُلُوغُ الْمَرَامِ
Bulugh al-Maram 431
The Book of Prayer
وَعَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا; { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ يَقْصُرُ فِي اَلسَّفَرِ وَيُتِمُّ, وَيَصُومُ وَيُفْطِرُ. } رَوَاهُ اَلدَّارَقُطْنِيُّ, وَرُوَاتُهُ ثِقَاتٌ. إِلَّا أَنَّهُ مَعْلُول ٌ [1] .
Narrated 'Aishah (RA): While on a journey, the Prophet (ﷺ) used to shorten the prayer (sometimes) and offer it completely (sometimes). Similarly, he used to observe fast (sometimes) and break it (sometimes). [ad-Daraqutni reported it, and its narrators are reliable, but it is a defective Hadith].
Sunnah.com reference: Book 2, Hadith 339