I know that generally you use the hiragana "わびさび", and in the entry for "侘寂" in the chinese wikipedia "侘び寂び" is listed as the original japanese, but I still have my doubts.
The appropriate spelling is 侘び寂び, even though 侘寂 is also understood by Japanese people, according to the discussion provided in this subreddit thread, (emphasis mine):
訓読み requires okurigana, so a word written only in kanji will be read in 音読み. However, there are some exceptions, including 侘 and 寂 in this case. For 侘 and 寂, there is 訓読み that does not require okurigaqna.This means that we can' t decide whether to read 侘 and 寂 with 訓読み or 音読み just by looking at them. So, in order to make the reading of kanji clearer, we often add okurigana, which should be unnecessary, such as び in this case. We've seen 侘び寂び with okurigana, not 侘寂, for a long time, so we recognize that 侘び寂び is the correct notation. As for kanji for wabi-sabi, we can say that 侘寂 is essentially the correct way to write it, but for modern people, it's a bit strange.
In the case of 侘寂, the meaning makes sense because we can easily imagine 侘び寂び, but we've never seen this kind of spelling, so we're uncomfortable with it.
For what is worth, 侘び寂び also appears as the main spelling for the corresponding entry in Jisho.org.
Finally, please check Why can some words be written with or without okurigana? How do the uses differ? to learn more about why sometimes an author may choose to change the regular okurigana of a word.