Context-free grammar how to have same number of variables within a language

I am trying to get a CFG for the language:

the set A of odd-length strings in {a,b}^* whose first, middle and last symbols are all the same 

(some example of correct answers would be: a, aaa, ababa, aababba, some incorrect answers include: ɛ, aaaa, abbaa)

This is what I’ve done so far:

S=a|b|aTaTa|bTbTb T=aT|bT|ɛ 

However the problem is, I need T to be a string of any combination of ‘a’s and ‘b’s but of the same length, but I’m not sure how to express this. As you can see above, I can get strings made up of any combination, but they won’t be the same length when passed to S. Any help is appreciated!