أخذ الحسن بن علي تمرة من تمر الصدقة فجعلها في فيه فقال رسول الله صلى الله عليه وسلم : كخ كخ ارم بها أما علمت أنا لا نأكل الصدقة (link)
Al-Hasan son of Ali took a charity date and put it in his mouth. So the Messenger of God, peace be upon him, said: "Kax, kax, throw it away; don't you know that we do not eat alms?"
Variants of this word (kxx, kexx, kexxa, kəxx) are very widespread in North Africa, not just in Arabic but in Berber too, as you can see from the Barefoot Linguist's Baby Talk database: it's used in Siwi, in Kabyle, in Tarifiyt, and in Senhaja. In Europe, on the other hand, it's far from universal; in fact, I don't know that it's even attested. That suggests that independent parallel innovation is unlikely. /x/ is a perfectly normal phoneme within Arabic, but in Berber it's rare in inherited roots and unlikely to be reconstructible for proto-Berber; all of the Berber languages listed there as having this word are intensely influenced by Arabic. That makes it unlikely that it's a common retention from proto-Afro-Asiatic. The most obvious conclusion is that kəxx has been borrowed from Arabic into Berber. Other cases of the borrowing of baby-talk is certainly attested, but this example seems particularly striking for the word's sheer frequency.