The entire prophesy is in 1 Kings 11:31-39.
The kingdom is taken "out of the hand of Solomon" (1 Kings 11:31) and ten tribes will be given to Jeroboam, while Solomon (in this case, his son, Rehoboam) will rule over the tribes of Judah and Benjamin (1 Kings 11:32). Solomon will continue to reign over united Israel for the rest of his days (1 Kings 11:34).
The reasons for the split:
Ahijah also prophecies that if Jeroboam serves God, that he will have a "sure house" (1 Kings 11:38).
When Solomon hears of this prophecy, he wants Jeroboam slain, so the future king flees to Egypt until Solomon dies (1 Kings 11:40).
Later on, Ahijah curses Jeroboam for turning to idol worship, and causing the inhabitants to do so as well (see Jeroboam for more details, or 1 Kings 14:7-16 for the full prophecy).