Halfway through the show, during the "unplugged" portion when Houston, 46, performs old hits, she announced it was her daughter Bobbi Kristina's 17th birthday, a source close to the singer tells PEOPLE. Houston then sang "Who Would Imagine a King" from The Preacher's Wife. "[It's] one of Krissy's favorites," the source says, "and is not in the show."
Then a birthday cake was rolled out on stage, and Houston invited her daughter to come up, joined by her dancers and another tune – "Happy Birthday," which was sung by the entire audience.
Mother and daughter also celebrated the occasion in Sydney earlier in the week during a stop on the singer's Nothing But Love world tour.
"They rented a beautiful boat and took a four-hour cruise all around Sydney Harbor with Krissy's friends, the dancers and family members," adds the source. "That was the pre-birthday celebration. The cake at the concert was a complete surprise to Krissy. She had no idea!"