It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to pray two rak'ahs during travel and four rak'ahs during residence. Therefore, whoever prays four rak'ahs during travel is like someone who prays two rak'ahs during residence. He also said that the shortening of the prayer happened only once, when the Prophet (peace and blessings be upon him) led two rak'ahs, which the Mujahideen performed as one rak'ah each, following the Prophet (peace and blessings be upon him) in congregation.