Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated: During travel the Prophet (peace and blessings be upon him) would sometimes fast and sometimes not fast, and he would offer two rak'ahs, which he would not leave. The narrator says: The Prophet (peace and blessings be upon him) would not add to them, meaning he would not add to the obligatory (prayer).
Imam Ahmad and Imam Abu Ya'la recorded this narration, and Imam al-Bazzar recorded something similar. The narrators of Imam Ahmad are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4908
Hadith Gradingمحدثین:إسنادہ صحیح
Hadith Takhrij«اخرجه ابو يعلى فى المسند (5309) اخرجه احمد فى المسند (407، 402/1) . اخرجه البزار فى المسند (992) ، أخرجه مسند أحمد برقم: 3803، وأخرجه إتحاف الخيرة المهرة بزوائد المسانيد العشرة برقم: 2354»