It is narrated from Hazrat Aisha (may Allah be pleased with her) that in the beginning, two rak'ahs of prayer were made obligatory; later, according to the command of Allah, the Prophet (peace be upon him) increased the prayer in residence, and left the prayer during travel as it was, i.e., two rak'ahs.
Hadith Referenceمسند احمد / مسند النساء / 25967
Hadith Gradingحکم دارالسلام:إسناده ضعيف بهذه السياقة لأجل أسامة بن زيد الليثي