Sarah Michelle Gellar is paying tribute to her late costar Nicholas Brendon in an emotional tribute ... saying "I know you are at peace."

The "Buffy the Vampire Slayer" star took to Instagram early Saturday morning to honor Brendon, sharing a throwback photo of the two and a quote that Brendon's character famously said on the hit show ... "They’ll never know how tough it is to be the one who isn’t chosen. To live so near to the spotlight, and never step in it. But I know. I see more than anybody realizes, because nobody’s watching me"

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SMG ends her tribute with "I saw you Nicky. I know you are at peace, in that big rocking chair in the sky."

As we previously reported ... Nicholas died per a family statement to The Hollywood Reporter saying, "We are heartbroken to share the passing of our brother and son, Nicholas Brendon. He passed in his sleep of natural causes. Most people know Nicky for his work as an actor and for the characters he brought to life over the years."

Brendon was a series regular on 'Buffy' opposite Gellar ... he played fan-favorite Xander Harris during the series' seven seasons. The reboot of 'Buffy' was recently scrapped over at Hulu, but it's unclear if Nicholas would have been part of the reboot.