Masruq says: I asked Sayyidah Aishah (may Allah be pleased with her) about the voluntary prayer of the Prophet (peace and blessings be upon him) during travel, and she said: There were two rak'ahs after every prayer.
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Sa'id bin Zunbur, whom Ibn Hibban declared thiqah (trustworthy).