When/where are chopsticks referred to as 箸子
箸 is used frequently in literatures of yore.

禮記﹒曲禮上　"飯黍毋以箸"

http://ctext.org/liji/qu-li-i/zh?searchu=箸&amp;searchmode=showall#result

numerous times in 史記

eg 史記﹒宋微子世家　"紂始為象箸﹒箕子嘆曰﹒彼為象箸"

http://ctext.org/shiji/song-wei-zi-shi-jia/zh?searchu=箸&amp;searchmode=showall#result

later, in 菽園雜記 of 明 dynasty, it recorded the rationale of people called it 筷子

"民間俗諱﹒各處有之﹒而吳中為甚﹒如舟行諱住﹒諱翻﹒以箸為快兒"

https://zh.wikisource.org/wiki/菽園雜記/卷01