Umayyah bin Abdullah bin Khalid bin Asid relates: I asked Sayyiduna Abdullah bin Umar (may Allah be pleased with them both): What is your view about shortening the prayer? We do not find it in the Book of Allah; we find mention of the prayer of fear. Umayyah said: Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: O my nephew, Allah Almighty sent the Prophet (peace and blessings be upon him) when we knew nothing, so we do what we saw the Prophet (peace and blessings be upon him) doing. Shortening the prayer is a Sunnah; the Prophet (peace and blessings be upon him) established it as a Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5388