The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1516
Ibn 'Abbas reported: The Messenger of Allah (may peace be upon him) observed the noon and afternoon prayers together in Medina without being in a state of fear or in a state of journey. (Abu Zubair said: I asked Sa'id [one of the narrators] why he did that. He said: I asked Ibn 'Abbas as you have asked me, and he replied that he [the Holy Prophet] wanted that no one among his Ummah should be put to [unnecessary] hardship.)