Be moderate in prostration, and let none of you spread his forearms on the ground like a dog

The Prophet (may Allah's peace and blessings be upon him) ordered that moderation should be observed in prostration, in such a way that the one praying adopts a good posture in prostration, with his hands on the ground and his forearms lifted and kept away from his sides. This is because such a posture indicates the energy and eagerness required in prayer, and also because this good posture allows all the prostrating organs to have their share in worship.

He forbade spreading the forearms in prostration because it is evidence of laziness and boredom, and it also constitutes imitating the dog, which is an improper imitation.