"They are more than friends, they consider each other sisters," a source tells PEOPLE of Francia Raisa and Selena Gomez

Donating an organ can be a big decision, but Francia Raisa was more than willing to give her best friend Selena Gomez her kidney.

Hours after Gomez revealed the news on Instagram, Raisa opened up herself on social media about her act of kindness for her friend.

“I am beyond grateful that God would trust me with something that not only saved a life, but changed mine in the process,” she captioned a photo of the longtime friends in the hospital. “This was part of our story, and we will share it soon, but what is important now is that this is not the only story.”

The actress also directed fans to the Lupus Research Alliance and added “Love you sis, so glad we’re on this journey together.”

“They are more than friends, they consider each other sisters,” a source tells PEOPLE exclusively. “They’ve seen each other through a lot over the years.”

Being a genetic match to donate an organ to a friend is not common, but the source says they are not surprised Raisa was able to donate to Gomez, who was in need of a kidney because of complications due to her lupus.

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“If anyone was going to be a match for Selena, it was going to be Francia,” says the source.

Gomez, 25, opened up about the transplant Wednesday morning a few hours after news broke about the procedure.

“Francia was more than happy to donate her kidney but hates that anyone ever found out,” says the source. “She wanted it to just be something she did quietly for her friend.”

“There aren’t words to describe how I can possibly thank my beautiful friend Francia Raisa,” Gomez wrote on Instagram of her friend, who will star on the black-ish spinoff grown-ish on Freeform in early 2018. “She gave me the ultimate gift and sacrifice by donating her kidney to me. I am incredibly blessed. I love you so much sis.”