Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1522
Ibn 'Abbas reported that the Messenger of Allah (may peace be upon him) observed in Medina seven (rak'ahs) and eight (rak'ahs), i. e. (be combined) the noon and afternoon prayers (eight rak'ahs) and the sunset and 'Isha' prayers (seven rak'ahs).