Abu al-Kanud says: I asked Sayyiduna Abdullah bin Umar about the prayer while travelling, and he said: "They are two rak'ahs that came down from the heaven (i.e. their command came down from the heaven); if you wish, you may reject them."
Al-Tabarani recorded it in al-Mu'jam al-Saghir, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2935