Some brothers go to the gym during their fast and work out for around an hour before breaking the fast. I read there is some benefit for the body / losing weight / etc doing this. I'm wondering if there is any evidence in the Hadith on this however?
I suspect this is just a practical constraint.
If we exercise during daylight, it may be too hot, and we can't drink to rehydrate until iftar. We may also need to go to work.
It might be impractical to exercise at night. E.g. the gyms are closed, people tend to sleep at night. Moreover, eating a large meal at iftar can make it hard to exercise afterwards.
And there's perhaps an I'm so hungry, I can't do anything that requires thinking period before breaking the fast.
There's indeed a benefit for the body in exercising, and it helps lose weight. But there's nothing Islam-specific about this.
Some "exercise during Ramadan" advice:
The best time to weight train during Ramadan is after taraweeh prayers at night. ... If this is too late then the next best time to weight train is about 1 hour after iftar before tarawih prayer. -- virtualmosque.com
If you can, get in about 25 minutes of exercise right before sehri/suhoor. This way you do not dehydrate your body while working out during you fast, and you are able to fuel yourself right away. However I realise this is not feasible for everyone, so another option is spending another 25 minutes just before iftar. -- islamgreatreligion.wordpress.com
Judging from various sites (1; 2; 3; 4) that talk about hadiths relating to exercise, there's nothing like what is asked for in the question. You can find things like Teach your children swimming, archery and horse riding (ref.) and A strong believer is better and is more lovable to Allah than a weak believer (Sahih Muslim 2664), but these don't relate to exercise before iftar.