"I won't leave you."
"You're safe."
Also don't over sympathise tears. Be careful with physical contact as this can be a trigger, and under flashbacks, panic attacks or breakdowns make sure it's okay to come near their body before doing so.
I'm just going to tell a story about when I got the best support ever. I was at an Iron Maiden concert with the bands Sahg, Kvelertak and Ghost as warm ups. I was in the front, but it went fine till Maiden got on stage. About halfway through the concert people started jumping more, and standing so close to everyone around me while people jumped up and down (made them rub against me) got unbearable. It triggered an anxiety attack, and I switched between screaming, sobbing and hyperventilating while I all the time shook and was in fight-or-flight mode. Fight was impossible; one person can't fight 20 000 crazy fans. Flight was also impossible; I was too scared to really move or turn around and the crowd was too tight. I ended up with lowering my head, lifting my shoulders and hiding in my arms because doing as I wanted and curl up in a ball on the ground would be a bad idea (mainly because people would pick me up -- metal fans are nice -- and that'd make me have their hands on me and yeah).
After about 30-40 minutes of panic I managed to walk further back among the people, because I knew some of the folks I went to concert with (friends of a friend) were not too far behind me. I found them, a girl and a tall boy, and she quickly grabbed my hand and lent over to ask me what was wrong. I just said "anxiety" into her ear and that was enough for her. We moved further away from the stage were people weren't standing that close and she held my hand and stood slightly before me by my side and he stood slightly behind me by my other side and let me cling onto his arm. She also said many times that "You're safe now."
That I could cling onto someone that accepted me and hold onto someone that supported helped so much, and being told I was safe and asked if I was better made me feel accepted and wanted. Holding onto them made feel closer to reality and safer. It's not what you say that's the most important, it's what you do.