Former President George W. Bush greeted former President Barack Obama with a tap on the belly as he arrived at Jimmy Carter's funeral service. A video of the interaction between the former presidents went viral, with tens of millions of views. Donald and Melania Trump were already seated with Obama when Bush approached, and Obama and Trump were seen chatting. Bush also exchanged greetings with former vice presidents seated behind him. All five living presidents gathered to remember Carter, generating several viral moments during the ceremony.
Source: https://www.huffpost.com/entry/george-w-bush-obama-belly-tap_n_677f8813e4b054f71f0bd44b