Sayyiduna Abu Huraira (may Allah be pleased with him) narrates that the Prophet Muhammad (peace and blessings be upon him) said: "When a person is performing voluntary prayers at night and reciting the Qur'an becomes difficult for his tongue, and he does not realize what he is saying, then he should go to sleep."
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2585
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (1184): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرطهما