The death of Fleetwood Mac's Christine McVie Wednesday at age 79 shocked many, including her bandmate and close friend Stevie Nicks.

“A few hours ago I was told that my best friend in the whole world since the first day of 1975, had passed away,” she wrote in the note posted to Instagram. “I didn’t even know she was ill ... until late Saturday night."

She said she wanted to travel to London to be with McVie but was told to wait.

Nicks wrote that since hearing McVie was ill, “one song has been swirling around in my head, over and over and over,” adding, “I thought I might possibly get to sing it to her, and so, I’m singing it to her now. I always knew I would need these words one day … it’s all I can do now.”

Nicks then included the lyrics to the third verse of the Haim song “Hallelujah,” which starts with, “I had a best friend but she has come to pass. One I wish I could see now.”

Nicks signed the note, “See you on the other side my love, don’t forget me, Always Stevie." She also added a picture of her and McVie together.

PHOTO: In this Jan. 26, 2018, file photo, honorees Christine McVie, John McVie and Stevie Nicks of Fleetwood Mac perform onstage during  MusiCares Person of the Year honoring Fleetwood Mac, at Radio City Music Hall in New York.
Kevin Mazur/Getty Images for NARAS, FILE
In this Jan. 26, 2018, file photo, honorees Christine McVie, John McVie and Stevie Nicks of Fleetwood Mac perform onstage during MusiCares Person of the Year honoring Fleetwood Mac, at Radio City Music Hall in New York.

Mick Fleetwood posted his own tribute to McVie, writing, "This is a day where my dear sweet Friend Christine McVie has taken flight.. and left us earthbound folks to listen with bated breath to the sounds of that songbird ... reminding one and all that love is all around us to reach for and touch in this precious life that is gifted to us."

He added, "Part of my heart has flown away today. I will miss everything about you Christine McVie."