Al Horford reached out to Hawks fans via social media on Tuesday, three days after he agreed to a free-agent contract with the Celtics.

Horford posted the following on Twitter Tuesday: “This was one of the most difficult decisions for me and my family, but I will cherish my nine years in Atlanta. From the bottom of my heart, to each and every Hawks’ fan, thank you.”

Horford, a four-time All-Star, was a cornerstone of the Hawks’ organization since he was selected with the third overall pick in the 2007 NBA Draft. He helped the Hawks to nine straight postseason appearances.

After being unable to come to terms with the Hawks on a new deal, Horford agreed to a four-year, $113 million max deal with the Celtics on Saturday. The deal cannot be signed until the end of the NBA moratorium on Thursday. Horford has spent time away with his family since the decision.