Glory to (Allah) who did take his servant for a journey by night from the sacred mosque to the farthest mosque whose precincts we did bless, ... Bani Isra'il 17:1
According to Muslim traditions, one night the Angel Gabriel woke up Muhammad, led him to a beast called the Buraq, "smaller than a mule but larger than an ass", and winged. Mounted on Buraq and with Gabriel alongside, he was transported from the Kaaba in Mecca to the Temple site in Jerusalem, and from there ascended to heaven. He encounters many of the earlier prophets and also meets with Allah and receives the instructions on how often Muslims should pray.
The claim of Muhammad's ascension to heaven (mi'raj) are purely a matter of belief. The reports about the isra', his the travel from Mecca to Jerusalem, however, are subject to historical evaluation. See these articles:
Based on the text of the Qur'an alone, it is even questionable whether it refers to Jerusalem, or whose journey it was! See the article, The Journey By Night, Ahmad and Zaid.
Similarities between Arda Wiraz Namag (Iranian "Divina Commedia") and the "Prophet's" Night Journey suggest that the story of Muhammad's Night Journey may be a pious invention patterned upon earlier legends from other cultures.
Further reading