Is sleeping on the left side forbidden because it is a characteristic of transgressors?

There is no evidence prohibiting sleeping on the left side. However, it is authentically reported from the Prophet (ﷺ) that he forbade sleeping on the stomach. Ibn Majah and others narrated from Qais bin Tikhfah Al-Ghifari, from his father, who said: "The Messenger of Allah (ﷺ) found me sleeping in the mosque on my stomach. He nudged me with his foot and said, 'What is wrong with you and this sleep? This is a sleep that Allah dislikes or abhors.'"