Which suffix (प्रत्यय) is used in the word 'कहावत' (Kahavat)?

A suffix (प्रत्यय) is an affix added to the end of a root word or verbal root to yield a new word.
Breaking down the noun 'कहावत' (proverb/saying):

$$\text{कहावत}=\text{कह (Root verb)}+\text{आवत (Suffix)}$$

Hence, the suffix combined here is आवत.