It's official, veteran NBA star Al Horford is signing with the Golden State Warriors. It was confirmed on Sunday that the one-time champion is joining the Warriors, a team filled with experienced players. After the news was made official, Horford's sister honored his legacy with the Boston Celtics.Anna Horford had a unique way of honoring her brother's stint with the Celtics. She had a collage of the memorable moments that the five-time All-Star had in Boston. She posted it on her Instagram stories on Sunday."Boston forever," Anna posted.Horford's sister honored his time with the Celtics on her Instagram stories.She also posted a photo of Celtics legend Paul Pierce on her Instagram stories. However, it wasn't just a regular photo of the 2008 Finals MVP, as Anna chose a hilarious picture of Pierce holding his tears.Horford's sister honored his time with the Celtics on her Instagram stories.Al Horford spent seven years with the Celtics and made a lasting impression on the fans and the organization. During his time in Boston, he was named a one-time All-Star and a part of the All-Defensive second team. In 2024, he won a championship with the franchise.In his two stints with the Celtics, Horford averaged 11.2 points, 6.8 rebounds and 3.6 assists. It's now time for him to move on from Boston and open a new chapter of his career with the Warriors.Al Horford sends heartfelt message to the Celtics following his move to the WarriorsBoston Celtics fans are emotional right now after it was reported that Al Horford won't return to the team. The big man was arguably a fan-favorite in Boston. Horford made sure to address the fans and the organization in his message."From the moment we arrived, you have welcomed my family and I with open arms. This chapter in my career will hold a special place in my heart. Raising banner 18 with this city will be a moment I will always cherish. I am forever grateful for the fans and the organization. Thank you, Boston," Horford posted.This isn't the first time Al Horford left Boston. He joined the Philadelphia 76ers ahead of the 2019-20 season after spending three seasons with the Celtics. Following that, he was sent to OKC and played one season for the Thunder. In 2021, Boston re-acquired him via a trade.Horford became an important part of the team's success during their 2024 title run. In that stretch, the big man was the team's veteran leader and averaged 9.2 points and seven rebounds.