Parashas Acharei Mos · Passuk 16:28
Chabad.org — English translation + Rashi
Torah Temima points to Yoma 68b.
English: One might think even after they have become ash they still convey impurity; “them” teaches that only while they remain recognizable carcass pieces do they render garments tamei.
Limit of tumah after burning
Once the chatat carcasses have been reduced to ash they no longer convey garment impurity.