It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) was asked about 'al-shaf'' (the even) and 'al-watr' (the odd), and he (peace and blessings be upon him) said: "(The even means) two days and a night: the Day of Arafah and the Day of Sacrifice (Nahr); and 'the odd' means the night of Sacrifice, that is, the night of Muzdalifah (Jam')."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4058