Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: When the Prophet (peace and blessings be upon him) travelled, he would pray two rak'ahs at a time, and when he was resident, he would pray four rak'ahs. The narrator says: Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Whoever prays four rak'ahs while travelling is like one who prays two rak'ahs while resident. The narrator says: Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: The prayer was shortened only once, when the Prophet (peace and blessings be upon him) prayed two rak'ahs and the people prayed one rak'ah each. (Allamah al-Haythami says:) I say: Part of it is in the Sahih.
Ahmad recorded it; its chain contains a narrator, Humayd bin Ali al-Uqayli. Al-Daraqutni (may Allah have mercy on him) said: He is not to be used as evidence; and Ibn Hibban (may Allah have mercy on him) mentioned him in the book al-Thiqat.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2942