To celebrate his first Father's Day [1] as a dad, Justin Timberlake [2] shared the sweetest picture of his little boy. On Sunday, the singer took to Instagram to post a photo of baby Silas, whom Justin and Jessica Biel welcomed in April [3]. JT is holding his son in the cute snap, with Silas sporting a onesie that reads, "I heart Dad." In his caption, Justin wrote, "FLEXIN' on Fathers Day . . . #HappyFathersDay to ALL of the Dads out there from the newest member of the Daddy Fraternity!! — JT." Check out other famous dads celebrating their first Father's Day [4], plus all the sweetest celebrity Father's Day photos [5].