Is there a prophetic Hadith that states: "I wonder at him who fasts and slept after Asr (afternoon prayer)?"

No authentic hadith has been reported from the Prophet (peace be upon him) prohibiting sleep after Asr prayer. The widely circulated hadith on this matter is fabricated, and lying about the Prophet is considered one of the major sins. The ruling on sleeping after Asr is subject to two opinions: The first is disapproval (karahah), based on narrations from the Salaf and medical experiences, but this does not constitute religious proof. The second is permissibility, which is the more preponderant view, because the default state of things is permissibility, and there is no authentic prohibition from the Prophet (peace be upon him). The statement of Al-Layth ibn Sa'd—who is one of the imams of the Muslims—supports this permissibility, as jurisprudence is derived from authentic hadiths, not from opinions. If medical harm is proven, it is disliked for a man to harm himself. However, from a legal perspective, there is no prohibition.