For stronger abdominal muscles, there is an exercise that is considered one of the most effective. It’s called “Russian Twist”. During exercise, both the oblique muscles, the straight as well as the lower abdominal muscles are trained. You do not need anything except your own body weight.
How to do it:
Take a medicine ball, a dumbbell or a weight plate and sit on the floor with your hips and knees bent 90 degrees. Keep the weight straight ahead and keep your back straight (your upper body should be about 45 degrees to the ground). As far as possible, blow your torso to the left  and then move the movement as far as possible to the right . This is a repeat, do 3 sets of 8-12 at the end of your workout.